Today we're going Greek! A questioner asks: Can you speak more on the Titans and imprisoning them in Tartarus? We can, and we so will.
The Titanes (male) and Titanides (female) - Titans in modern English - are the gods of Greek mythology to whom its most ancient and archaic rites were dedicated and who preceded Zeus and the reign of the Olympians, creating most of the world and ruling over it absolutely until the coming of the younger gods. Their name is so old that its etymology is uncertain, but Hesiod claims it comes from the root word meaning "to extend", referring to the fact that the Titanes easily covered and exerted their influence over all of the universe. They are seldom described as a group, more often as individuals, but they are often considered to have been enormous in either a physical or metaphorical sense (or both), which has led to English using the words "titan" and "titanic" in the modern day to mean something really, really big.
The Titanes as a group are the only real opposition to the gods in Greek mythology, which should not be surprising since they are in fact gods themselves. In fact, while Greek myth makes a distinction between the titanes, the old gods, and the theoi, the new gods, they are all at the end of the day gods; it's a generational difference, with the Titanes being the older generations that created the world and gave birth to the popular gods of today, rather than one of any actual difference in divinity. The Titans became the enemies of the gods after Kronos, the Titan god of harvest and prosperity, devoured all of his children except for Zeus in fear that they would overthrow him one day; Zeus' mother Rheia saved him and when he became an adult he forced his father to disgorge all his siblings, after which war was waged between the two sides. Kronos' prediction came true when Zeus did eventually defeat the forces of the gods and take the thone, hurling his father into the underworld prison of Tartarus, just as Kronos had once overthrown and castrated his own father Ouranos.
Because deities are truly immortal in Greek mythology - they can be wounded, put to sleep, inconvenienced or angered, but they can never be killed - the gods were presented with the unique problem of what to do with an entire army of immensely angry primordial beings that they couldn't defeat by killing them and that would never run out of time or energy to oppose them. They settled upon imprisoning the Titanes in Tartaros, a region of the underworld Hades described as a great bottomless pit filled with storms into which the Titanes could be hurled to prevent them from returning. In early Greek cosmological thought, the universe was considered to be a sphere, with the vault of the sky forming the top half of the circle and the underground pit of Tartaros forming the bottom half, so imprisoning the Titanes in Tartaros was also pleasantly symmetrical, leaving the heavens to be Zeus' domain and under the general ownership of the younger gods. It's interesting to note that Tartaros is a Titan itself - an extremely ancient and primordial one, similar to Gaia who was also the earth and Ouranos who was also the sky, and though it is seldom personified enough to take actions among the gods, it does have some children of its own.
There are various famous Titanes in Greek mythology, many of whom had their own worship and cult followings, although the later into Greek religion you get, the more they become background characters rather than figures of worship in their own right. The original Titanes, the Protogenoi, were the primordial creators of everything and lords over the universe when there was scarcely a universe to lord over; these include beings that are so enormous and ancient that they are hardly personified very often, including Khaos, Tartaros and Erebos, as well as more active and vibrant characters that interact with other gods and are spoken of in ancient stories, such as Gaia, Ouranos or Okeanos.
Following them, the most famous of the Titanes are the twelve who opposed Zeus (a mirror of the twelve Olympians now in power in the Greek pantheon), which included six female gods and six male, who represented the ancient powers of divination and foresight as well as the pillars that support the universe. These were Koios, the Axis of the Heavens around whom the stars swirl, and his wife Phoibe, first of the oracles after whom Apollo and Artemis were later named; Krios, lord of the southern stars and master of rams; Okeanos, lord of the primordial oceans that circle and support the world, and his wife Tethys, the nurse of the gods and mistress of oceans, springs and waters; Iapetos, limiter of the lifespan of all mortals and wester pillar of the heavens; Hyperion, the first lord of the sun and all other heavenly lights, and his wife Theia, lady of the shining blue sky through which light must come; Mnemosyne, the mistress of memory who records all of existence; Themis, the goddess of divine law and order whose commands can never be contradicted; and Kronos, god of good harvests and the cycles of the seasons, along with his wife Rheia, mistress of motherhood, creation and female fertility.
Although these were the twelve opposite points to the twelve gods of Olympus, they weren't all involved in the war against them; just like the younger generations of gods, the Titanes do not always agree, back one another up, or avoid irritating one another when it serves their purposes. In particular, Mnemosyne took the side of the gods (and later became the mother of the Muses by Zeus, in some versions of their parentage), and Okeanos and Tethys chose to remain neutral, with the latter nursing the gods in place of their mother once they were rescued from Kronos' belly.
However, even past the twelve Titanes and their forbears the Protogenoi, there are tons more Titanes, many of whom have their own functions in ancient Greek religion and were far from forgotten after the worship of the younger gods became popular. Among them are such figures as Helios and Selene, gods of the sun and moon who were later succeeded by Apollo and Artemis but never quite forgotten, along with their sister Eos, goddess of the dawn who mourned over her fallen son at Troy; Hekate, the three-faced goddess of sorcery and crossroads, called upon in the same breath as Hermes to give travelers safe passage on their journeys; Prometheus, who robbed the gods of their sacrifices and stole fire to give it to humanity, earning him Zeus' displeasure forevermore; Nyx, goddess of the primordial night, and her sons Hypnos and Thanatos, bringer of sleep and death, and many, many more. Other gods who were probably once considered Titanes or Titanides were repurposed later in the religion to become part of the stories of the younger generations as well - for example, Amphitrite, who in later tales became the sea-nymph wife of Poseidon but who was most likely the primordial goddess of the Mediterranean long before his worship became popular.
So what does all this mean for Hero's Journey? Well, it means that the Titanes are a powerful force in Greek religion and mythology, and may affect games exploring Greek themes in various ways. The most contentious among the Titanes have already been defeated and imprisoned in Tartaros, but those remaining may still resent the undue control of the younger gods, particularly if they feel they are not being properly respected now that worshipers have largely turned away from them, so it's still possible to use them as antagonists against Heroes or their divine patrons if your game so chooses. Many of them are also quite actively worshiped with their own temples and rituals around the world, so while they aren't presented as divine patrons in the core rulebook, they could certainly call upon Heroes of their own and become driving forces behind the adventures of your players instead of enemies to array against them. And of course, should your Heroes be interacting with gods, the Titanes are possibilities to worship, call upon or interact with just as much as any other Greek god - even some of those conquered long ago, if you visit their prisons (or believe Hesiod, who claims that some of them were let out for good behavior after they'd served an appropriate number of eons locked away in Tartaros). Tartaros itself is a place that could be visited by Heroes, although we would not recommend going voluntarily into it except in the direst of need.
The Titanes and Titanides may be old, but they are no more evil or dangerous than any other Greek god, and Heroes who pay them the proper respects may be able to discover exactly how much weight earning the favor of one of these ancient gods can be worth on their adventures.