Oh, man, I am so excited about this request: Here's a concept I'd like to hear more about: Sacrifices to the gods! Seriously, I've been saving it up in the question box for a week or two just so I could make sure I had time to do it justice.
Sacrifice is a core concept in religions and myths around the world; it's what anthropologists like to call a cultural universal, meaning that it appears in some form almost everywhere on the planet. Every human society has done it at some point for some important purpose, and it's a core feature of almost every religion in existence in one dimension or another. There are a thousand classifications of different kinds of sacrifice, all of them equally solemn and all of them ways that humanity calls upon to be able to interact with the divine; in essence, sacrifice is a religion's way of allowing humans, in some small way, to control the universe by doing something that will have a guaranteed result. Sacrifice lets us give something and know that something else will be given in return, and thereby give religions and people power even when they live in a universe full of unfathomable divine forces above them.
Sacrifice isn't just about cause and effect, though; its very core concept is that whatever the worshiper gives up to a god, it must be significant and it must impact the worshiper in some way. A sacrifice that is easy to lose or doesn't matter to the person who gave it up isn't a sacrifice at all, because it follows that the god is not being truly addressed or honored by it and would find it equally inconsequential. Almost all religious sacrifices, even in cultures where it became fairly commercialized (ancient Rome, for example), carry a heavy element of giving up something valuable or meaningful, which is what elevates the ritual from simple commerce for favors to an act of religious devotion. Sometimes it must be something that is valuable to the god themself; sometimes all that matters is that the worshiper found it valuable enough that giving it up proves their devotion.
There are a million reasons for sacrifice, but the most common fall into the general categories of connection, supplication, reverence and protection. Connection sacrifices are performed to link the human making the sacrifice to the divine, allowing humanity to touch the gods for a few moments; such rituals are usually intensely personal and are enacted in order for someone (often a shaman, priest or other designated religious leader) to break the barrier between worlds for a little while and accomplish important tasks or learn important information. Supplication sacrifices are possibly the most common around the world; they are sacrifices given to gods in order to ask them for their favors, which might be anything from granting worshipers prosperity to defeating their enemies, and work on the theory that if worshipers give to their gods, the gods will give in return. Reverence sacrifices are instead concerned with granting the respect to deities that a given religion believes is their due - they are performed because gods require praise and reverence, and because that's humanity's job, which some religions take extremely seriously. And finally, protection sacrifices are performed to stave off disaster or prevent a deity from becoming angry; these appear most often in cultures whose religions are built around unpredictable, dangerous or unfriendly deities and spiritual forces, and are intended to keep those powers calm, happy and unlikely to lash out at humanity.
Sacrifice specifically to gods helps narrow the field a little bit (it takes away other forms of sacrifice, such as sacrifices performed to better oneself without divine aid or sacrifices made to aid the community in a non-religious context), but even so it's a wide spectrum of possibilities. Anthropologists, mythographers and sociologists can spend their entire careers on the subject of sacrifice and never cover all its possible permutations and all the things it can and does mean to various cultures throughout history. The person who sent in the question asked that we focus specifically on the four pantheons in the Hero's Journey core rulebook, which is a reasonable request, but I am TOO EXCITED to talk only about them. I'll try to focus down when needed, though.
Like I said, there are so very many different kinds of sacrifice as a ritual concept, but the most common in myth and religion tend to fall into these categories:
Animal Sacrifice. This is one of the most common kinds of sacrifice you'll find in world religions, especially polytheistic ones, and it becomes more and more common the further back into history you go. In religions that practice animal sacrifice, an animal is killed in a ritual manner as an offering to a god, after which various things are done with its body according to the religion's specifications. Sometimes the body parts are used in further religious or magical rituals, which is often the case in West African religions; sometimes the animal is then ritually eaten by the worshipers who sacrificed it, common in Mediterranean and Semitic religions, which allows the people who participated in the ritual to share in the sacrifice with the god and therefore be bound closer together; sometimes the animal is preserved indefinitely to illustrate that it belongs to a god now and can no longer be touched, which is often the case for the animal mummies found in various Egyptian gods' temples; and sometimes the animal's body is completely destroyed, in order to show that it has left the realm of humanity or to ensure that no one else attempts to steal any part of it now that it has been commended to the divine. Occasionally, animal sacrifices don't involve killing an animal specifically for a god, but rather offering part of each animal killed for sustenance to a deity before taking any for oneself, therefore making all animal death for any reason part of the religion's sacrificial rites.
Animal sacrifice has several important symbolic meanings; in the first and probably most ancient place, it's important as a sign of devotion because animals represent food and livelihood, so giving one up to a god was a real sacrifice for ancient cultures, who would no longer have that animal's meat, milk, eggs, or any other product it could have given them. In times of drought, famine, harsh winters or depredations by other nearby peoples, having enough animals to maintain herds and feed people was critical, and giving any of them up therefore an act of real devotion. For some cultures, the gods were considered to sustain themselves on sacrifices or at least to enjoy eating just like everyone else, so sacrificing an animal to a god was literally feeding them just as its flesh could have fed a human, something that they would certainly appreciate and in some cases even actively needed from humans. Some religions also sacrificed animals to specific deities because of a religious idea of "belonging"; that is, a god might receive sacrifices specifically of their totem animal, because all specimens of that animal were considered to belong to them by default, so human worshipers are simply performing the role of errand-runner by delivering them to them (as well as currying some favor for being sensitive to a god's specific preferences). In this last case, some animal sacrifice wasn't even lethal to the animal; for example, ancient Arab religion sometimes involved a community giving up a prize bull to Dushara, god of the mountain, which meant that the bull would be set free on the mountain to roam for the rest of its life, and that no one could touch it or eat it for fear of the consequences of stealing a god's property.
Ancient Greek religion (see, I said I would focus!) performed animal sacrifice often and for most deities, and based its practices around the idea of sacrifices being sustaining for the gods and being something that humanity owed them for being so awesome and powerful in the first place. Failing to sacrifice properly or often enough meant that you weren't appropriately respecting the Greek gods, which I'm sure we all know is the first step on the road to infinite smiting, and even those too poor to give up the one goat the family owned could still attend temple sacrifices and participate in the rituals in the hopes of showing their devotion. The sacrificed animal (usually cattle, sheep or goat) was also often shared as food among the worshipers present - in some cases, just a mouthful, in others, such as the giant festival sacrifices to Athena, in a huge community feast. The animal's bones and skin were most often burned (the flesh, too, if it wasn't eaten), in order that it should be destroyed on earth and reach the gods; fire is often used as a sacrificial conduit in various religions because of its ability to destroy things and the idea that its smoke, rising up to heaven, might reach the deities that dwell above humankind.
Animal sacrifice was also very common in historical Hinduism, although it has become less so in modern times; many branches of Hinduism now forbid it, and those that do practice it most often do so in sacrifices of goats or chickens (historically, even horses and elephants) performed by priests or nobility to specific deities who are particularly associated with sacrifice. Kali, for example, still receives fairly regular animal sacrifice in some parts of India thanks to her warlike qualities and mythic connections to blood and death. Unlike Greek sacrificial practice, the meat of animals sacrificed to the gods is usually not eaten by worshipers, who consider it to no longer belong to humanity, and the animal is often burned in order to send it to the god it is dedicated to as well as destroy it so that it can be reborn into a new life in thanks for its service.
Blood Sacrifice. On the other hand, in spite of the fact that we tend to think of it first when we hear the term "religious sacrifice", blood sacrifice is comparatively pretty rare around the world. This is because it requires a more complicated and less universal set of religious ideas to support it: either there must be something specifically about blood that makes it valuable as a sacrificial substance, which is the case for many Mexican religions in which blood is believed to contain literal power and energy and therefore to be a needful offering to the gods, or the act of shedding blood (or more specifically, causing hurt to oneself) must be religiously important, which generally occurs in religions that believe that suffering proves worth or provides insight or enlightenment.
Blood sacrifice in the big, showy, spectacular form of slitting someone open and letting them bleed everywhere is generally an American phenomenon; the Mexica religion and those of nearby other peoples are heavily based on the idea that the gods need energy in order to keep the universe in order and away from destruction, and since blood was believed to contain the primordial energy they used, blood had to be spilled to provide it. Depending on the culture, blood sacrifice can occur in conjunction with either animal or human sacrifice; for example, in the late period of ancient Rome the taurobolium was practiced, a religious ritual in which a bull was killed and slit open in order to let its blood pour onto a ritual participant. (You can watch HBO's Rome do a dramatization of the taurobolium here, but don't if you don't want to see someone get an entire bull's worth of blood poured all over them.) Human bloodletting is performed in various religions not necessarily because blood itself is considered powerful, but because a worshiper being willing to hurt themselves is an act of obvious dedication and therefore has religious power all on its own. The sacrifice itself is what's important here, not the blood, which is just a symbol that represents the sacrifice and the pain that was incurred to perform it. This kind of blood sacrifice is fairly common in Celtic religions, which is a large part of the reason that modern pop culture associates the idea with sorcery and witchcraft. Finally, blood sacrifice can also be performed as a means to an end; by causing oneself enough pain or blood loss, a person might be able to receive religious visions or prove their worth to be contacted by a god, something they might have to approach the very brink of death to achieve.
Interestingly, blood sacrifice, especially for men in various cultures, often revolves around the penis. Often, letting blood from the penis is considered more hardcore than doing it anywhere else on the body (for obvious reasons) and it bleeds more easily; and for many cultures, including the Maya and several indigenous Australian peoples, such rituals are considered to give men the ability to briefly access the innate powers of creation that women already have by bleeding from their genitals and in essence duplicating the female ability to menstruate.
To get more specific again, ancient Norse religion practiced a wide variety of different kinds of sacrifice, but blood sacrifice was one of them in spite of its general rarity in Europe. Their sacrificial rites were called blot, a word that has the same root as the modern English blood, and included not only animal and human sacrifices but also blood sacrifice, in which blood spilled for sacrificial purposes was believed to contain magical properties and to confer power, and which was ritually painted or sprinkled on statues of the gods to symbolically grant that power to them.
Human Sacrifice. Human sacrifice is also more rare than animal, but it's also very widespread in historical religions; the further back you go, the more human sacrifice you'll see, even in religions that are still alive now but have ceased to perform it somewhere down the centuries. Human sacrifice is complicated, because the reason for it can be wildly different in different cultures. Some considered it a necessary part of placating their gods, and believed that the gods would take some of humanity whether they wanted it to happen or not, and that therefore sacrificing people ahead of time could give the community the power to decide who was sacrificed and prevent the gods from being angry or taking a large number of people (similar to the fairytale convention of sacrificing a princess to a dragon rather than waiting around for the dragon to attack the town and kill a hundred other people). Information on mainland Celtic religions is spotty, but this is likely the reason behind their occasional human sacrifices, and the reason that many of the sacrificed appear to have been criminals that the community would be willing to give up. On the other hand, sometimes it is not the criminals or old people who are sacrificed, but the youngest and most beautiful; the theory behind this is usually that the gods would object to being given a substandard sacrifice, so only someone perfect or desirable will do.
Other religions, including that of the Inca people of Peru, practiced human sacrifice in order to connect the people with their gods; humans were given in death to the gods and spiritual forces of the universe to demonstrate their devotion and basically give them a man on the inside, so to speak, when that sacrificed person became part of the power that animated the landscape and decided mortal fates. These kinds of human sacrifices are extremely important and solemn affairs, and represent an entire community choosing a representative from among them to be the communicator to the gods or even to walk with them, something that is never taken lightly. This is another kind of sacrifice that doesn't necessarily have to be lethal, too; giving up children to become members of the clergy or to serve a specific religious function is also a form of human sacrifice, one that families must make in order to put their religion and the good of their community above their own desire to remain with their loved ones. And finally, in cultures that place a specific value on human life that is above that of animal life or material objects, the sacrifice of a human being becomes the most potent of all sacrifices specifically because it represents giving up the most important and sacred thing a human has - their life - to the powers that be. This is comparatively pretty rare for purely sociological reasons; as societies evolve toward considering human life innately valuable, so they are also usually evolving away from the idea of killing people being acceptable, even in a religious context.
Although again most of us tend to associate the idea of human sacrifice with only a few select mythologies (especially Mexican and Hawaiian, both of whose practices were not-so-coincidentally exaggerated in order to make invading European cultures look better for putting a stop to all that), it has been documented occuring in almost every religion at some point in its history, from widespread practice to occasional but important sacrifices. Ancient Norse religion is particularly heavy on human sacrifice, with hanging and drowning sacrifices particularly possible in order to ask for favor from specific gods - offered to Odin for success in battle, for example, or to several of the Vanir gods for good harvests or prosperity. Human sacrifice was rare in ancient Greece but not unheard of, and in fact Agamemmnon sacrifices his daughter Iphigenia to Artemis in order to get from the goddess clear passage for his army across the sea to the Trojan war; this is not condoned by the text and in fact Agamemmnon gets his ass kicked for it later on, but it's hard to tell whether it's the fact that he committed human sacrifice, the fact that the victim was his blood relation (Greek society was very unforgiving of familial violence) or the fact that Iphigenia may already have been dedicated to or beloved by Artemis in some way and therefore the goddess was offended by the choice. Whatever the reason, however, the story still lets us know that human sacrifice was known and occasionally practiced, and that it did in fact work to secure Artemis favor (however capricious and temporary). Human sacrifice was not as common in ancient Indian religion as in some others, but it, too, existed, and was most often practiced to honor particularly warlike gods or to ask for divine aid in especially dangerous times, such as when war or plague threatened the population. Ancient Egypt's is one of the few religions that didn't practice human sacrifice to their gods much (although we do have some isolated evidence of it around the first dynasty and earlier, so it's likely that they did but simply stopped the practice further back in history than most other religions), but the practice of killing servants and retainers in order to send them to accompany and serve important nobles or the pharaoh after death remained common.
Object Sacrifice. This is by far the most common form of religious sacrifice in the modern day, but it's been popular throughout the ages. Object sacrifice involves giving up a thing that is precious to oneself; like some forms of blood sacrifice, it's often not really about the object that you're giving up, but rather about the act of sacrificing it and a person's ability to prove their worth or devotion by doing so. Object sacrifice is incredibly varied around the world - foods, currency, fetishes, personal household items and more are all in various places considered viable options for religious sacrifice.
Sometimes, object sacrifice is about actually giving something to the gods or spirits for their own use; for example, the Norse practice of throwing away unused boot leather is a kind of sacrificial act to "give" that leather to the god Vidar, who will use it to create the giant boot he will wear at Ragnarok to destroy the great wolf Fenrir. Sometimes the god simply takes ownership of a sacrificed item when the act of sacrificing it makes it off-limits, such as when Inca kings would throw gold overboard into Lake Titicaca to appease the deity there; no one would dream of swimming down there to retrieve it, because that would be theft from a god, never a good idea. In other cases, the sacrificed item becomes communal property, sometimes meaning that everyone uses it and the religious ritual is intended to foster community harmony, or sometimes that a religious official administers it and gives it out only to those deemed worthy. Food sacrifices are especially common; sometimes, like animal sacrifices, they are believed to actually be feeding a god (for example, butter is used in Hindu sacrifices because it's well-known that Agni, god of fire, likes butter and will enjoy licking it up while he burns up the sacrifice), or at other times the food is simply thrown away as a sign of respect to the deity who provided it or allowed mortals to harvest it, such as when in Slavic lands the first glass of wine or bite of any mean is poured out on the ground to honor the earth-mother Mokosh. Clothing sacrifices are sometimes intended to literally clothe a god - for example, many god statues in India are ritually dressed in fine clothing to honor them - while in other cases clothing is sacrificed to become part of a temple's holdings, and is thereafter used for god impersonations or for the purposes of the clergy who support that god's religion. And, as is the case with animal sacrifice, sometimes objects are given to gods simply because they are already considered to "own" them and therefore it's a sign of respect for humans to recognize that, such as when the Mexica goddess Xochiquetzal is given sacrifices of flowers to acknowledge that she is the mistress of all things floral.
Because object sacrifices involve concrete and valuable items, and because those items, with the exception of those cultures that actually destroy them during the process of sacrifice, are often still in existence after the ritual, it's common in a lot of religions for the priests or shaman involved to make use of them themselves. Sometimes this is approved by the religion itself, often because the holy figure is considered to represent their god and therefore their using it is the same as the god using it, and the practice is encouraged as a way of the community supporting the clergy as well as honoring the gods. Sometimes, the priests are considered to be no more in possession of sacrificed items than anyone else, so taking or using them is severely frowned upon.
Personal Sacrifice. Personal sacrifice requires a religion to be invested in the idea that the personal beliefs, ethics or private actions of an individual are just as religiously important as anything else; that is, the idea must be present that a worshiper's personal thoughts and desires are part of their religious life as much as anything else. This is a newer concept in the general landscape of world myth and religion, which in older times mostly considered what a person did to be the religiously relevant part of their lives, not what they might be internally thinking or feeling. It's especially important in religions that emphasize spiritual growth as part of their philosophy, because it can be used as a method of forcing a person to change or test their personal limits through difficult sacrifice that doesn't necessarily require any outside help or cause impact to others. Religions that emphasize enlightenment or escape from physical concerns such as Hinduism and Buddhism often have branches that heavily espouse personal sacrifices, as well as them being a major component of many monotheistic religions.
Personal sacrifices often involve privation - the person performing them sacrifices something they normally want or even need and must survive without it, which often includes fasting, abstaining from alcohol, sex or certain foods, or otherwise placing restrictions on their behavior that make their lives more difficult or less comfortable. These sacrifices, when connected with a god, are almost always performed in order to prove the worshiper's devotion and convince the god to either bestow favor upon them or refrain from punishing them, if the sacrifice is performed as part of a penance. In some religions, personal sacrifices are thought to in return confer upon a person spiritual power - for example, in Shintoism, in which priestesses remain celibate not because the religion forbids them (for most people, Shinto discourages celibacy because it prevents living a balanced life) but because this personal sacrifice renders them able to conserve and reuse purifying energy in a way that normal people cannot. In other cases, a deity might demand a personal sacrifice of their worshipers; for a particularly grisly example, Cybele's worshipers castrated themselves during some of her rituals, a very concrete and frightening form of sacrifice. (And, as that example makes clear, personal sacrifice can and does overlap with human sacrifice occasionally, although more often it's about sacrificing something in your life or personality, not your body parts.)
Service Sacrifice. And finally, service sacrifice, which is the practice of dedicating oneself to a particular task, service or duty and sacrificing all other occupations you might have preferred. Many religions gather their clergy and religious practitioners this way; becoming a direct servant of the god might mean having to sacrifice your home or family life in order to live and participate in religion somewhere else, sacrificing your aspirations to do anything else in favor of performing as a religion requires you instead, going on a sacred crusade or mission, or even marrying someone you don't want to because it's the right thing to do in the eyes of the religion. Service sacrifice always involves deciding that what you want to do isn't as important as doing what the religion requires of you, and is often a matter of ongoing action and dedication rather than a one-time gift of an object or animal.
This is harder to pin down as a religious concept sometimes, simply because it can overlap so much with the general idea of piety; people perform religious rites, attend religious ceremonies and attempt to conform their behavior to their religion's demands all the time, and any of these things could technically be called a form of service sacrifice, although whether it's done for a god or only for a person's individual well-being or growth depends on the religion. Simply giving up the time and effort required to do something for one's god is a form of sacrifice, whether it's as extreme as becoming a monk for the rest of your life or as occasional as attending an hour-long temple ceremony on major holidays.
This post is definitely already long enough - like, so long I feel like apologizing to those of you who follow the blog on your RSS feeds and got this monstrosity in your inbox today - but I still feel like it's important to talk a little bit about sacrifice in Hero's Journey now that we've taken that magical mystery tour through historical sacrificial practices. As we've noted above, sacrifice occurs in every religion in some way, and the living religions of HJ will certainly practice it in some dimension; it'll be a major part of the religious world, although in what way varies depending on the religion and the area in which it is practiced (you're much more likely to get a traditional pig sacrifice in rural Sweden than in London, for example, even though both areas have strong Norse religious communities). Heroes will be likely to run into all kinds of examples of sacrifice, and will have to decide which they practice themselves and what ethics they may need to apply to them.
As for sacrifice as a mechanic, that's something that Heroes will also get to play with, but just as the idea is way too big for a single definition, so it's too big to be described by a single mechanic. Different kinds of sacrificial action may appear in various Blessings, especially in the Devotional Domain, allowing Heroes who represent different pantheons to tap into the specific religious practices dedicated to them.
And I think we all need a break now, so see you tomorrow!