We're straying a little far afield from the beginning HJ pantheons' myths to head back to Mexico with this question from the submission box: How about some information on those terrible ladies of the Aztecs known as the Tzitzimitl?
People who have been following me around on the internet for a while probably already know about my special interest in Mexican mythology; I am a big fan of Mesoamerican mythical stories and religious rites, which are way fascinating and don't get nearly the attention and scholarship they deserve, and the tzitzimime (singular tzitzimitl, plural tzitzimime) are one of their most complex and interesting features. They are a class of spirits or lesser deities in Mexica myth that are poorly understood, even now, but generally portrayed as frightening and grotesque in surviving accounts and artwork.
One of the major issues with the tzitzimime is the fact that we don't actually know very much about them, and what little we do know comes to us through very suspect Spanish sources. In artwork, they're presented as skeletal and terrible-looking, with insectoid attributes including claws and occasionally wings that align them with concepts of death and danger, and are usually depicted with blood, tying them to sacrifice and warfare. But beyond that point, we're caught between trying to figure out what the tzitzimime really did and represented in Mexica mythology, which we have to piece together from artwork and a precious few pre-Conquest sources, and what they were transformed into by the interpretations and revisionism of the Spanish friars and recorders who wrote about them through the filter of their foreign culture and religion.
According to the stories and religious texts recorded by Spanish writers like Sahagun and Duran, the tzitzimime were demons, heavily masculine forces for evil that devoured humans, spread plagues and were constantly engaged in attempting to destroy the sun god Huitzilopochtli and end the world. The fearsome appearance of the tzitzimime caused the Spanish to equate them with devils, and the fact that the Devil of Catholicism is male, combined with iconography of serpents and dangling clothes that they mistook for phallic symbols, convinced them that these were essentially male creatures, which caused them to distort stories about the tzitzimime by conflating them with Christian images of the Devil and his minions.
However, the earliest recorded stories about tzitzimime refer to them as female beings, and now that scholars have expertise in Mexica iconography that the Spanish didn't centuries ago, they can identify the clothing and images associated with artwork of the tzitzimime as feminine in context of Mexica art. It's more likely that these were originally a class of feminine deities or creatures, which is further suggested by their frequent comparison to the cihuateteo, another group of fearsome female deities who were believed to be created from the ghosts of women who lost or outlived their children. Both tzitzimime and cihuateteo are often invoked alongside one another, and the leaders of each order of demons - Itzpapalotl, the Obsidian Butterfly who leads the tzitzimime, and Cihuacoatl, the Serpent Woman who is the archetypal member of the cihuateteo - are sometimes used interchangeably with one another in mythological texts.
In addition to probably being considered female in their native religion, the tzitzimime were most likely a lot less demonically evil than the friars later considered them. They were definitely considered dangerous and frightening; they are associated with death and illness, which they were sometimes called upon to stave off (especially by midwives assisting with birth and other female-specific ailments) and sometimes feared to cause. They were also associated with the stars; Mexica mythology places the sun in the position of paramount importance and revolves around the idea that the sun fuels and supports the entire world, which therefore causes beings and elements associated with the opposing nighttime to be viewed as suspicious and potentially dangerous. Especial mention is made of them representing the stars during the period of a solar eclipse - among the most dangerous and unstable times in the universe, when the sun might not return and the world could plunge into destruction - and even if the characterization of the tzitzimime as devils is partly rooted in Christianity, it's clear that they were a force to be feared in their native religion well before the introduction of monotheistic conquerors.
In spite of being terrifying people-eaters, however, there are also hints that the tzitzimime have their place in maintaining the running of the cosmos, like all other Mexica deities. One theory is that they do threaten the sun and the sun deity that supports it, but that they do so in order to keep it moving through the sky and bringing life to the world, similar to the myth in which the god Tlahuizcalpantecuhtli shot an arrow at the new embodied sun Tonatiuh, not to destroy it but to force it to begin nourishing the world as it was intended to do. After chasing the sun across the sky and taking over the heavens for the night, they are then defeated and devoured by Huitzilopochtli to recharge for the day. As important (if thoroughly scary and unpleasant) parts of the working of the cosmos, they require a constant stream of blood and sacrifice - and if they don't get it, they'll find it on their own.
Myths in which the tzitzimime directly appear instead of just being mentioned as folkloric creatures are few; they aren't major characters like the most common Mexica deities, and as dangerous and terrible creatures are not usually appearing as positive forces in anyone's stories. But there are a couple of them, both of which are neat windows into the way the tzitzimime appear in Mexica myth.
The first story revolves around Mayahuel, the goddess of the maguey plant and its important products (including thorns, used for weapons and discipline, clothing and paper made from its leaves and alcohol made from its fermentation), whose grandmother is one of the tzitzimime. According to the story, Mayahuel was very beautiful and caught the eye of Quetzalcoatl, god of the heavens, who fell in love with her and convinced her to run away with him. The lovers hid in a tree to sleep through the night, but when Mayahuel's grandmother discovered that she was missing, she summoned all the other tzitzimime in a rage and flew down upon the earth to find the missing goddess. Quetzalcoatl was able to see them coming and flee in time to avoid them, but Mayahuel was torn to pieces by the star creatures, after which those parts of her that weren't devoured fell into the ground and grew into the first maguey plants.
There's a lot going on in this story; to begin with, Mexica culture restricted female sexuality very strongly, so Mayahuel's tragic end is a result of her having an illicit relationship with Quetzalcoatl, which she is immediately punished for in the form of her family's angry and evil backlash (and on the flip side, while such behavior wasn't encouraged in men, it was punished less harshly, which is illustrated in Quetzalcoatl's ability to escape the same fate). Also, while Mayahuel's grandmother isn't named in the myth, her apparent ability to summon all the other tzitzimime to do her bidding suggests that she may be none other than Itzpapalotl herself, the cosmic creator of the world and mistress of the beings that constantly threaten it.
The other major mention of the tzitzimime in Mexica mythology comes from Martin Ocelotl, a native Mexica priest who waged a quiet but defiant campaign to resist his peoples' conversion to Christianity by the invading Spanish and continue to interpret the world through a native Mexican worldview. One of his proclamations was that two indigenous prophets ("apostles", according to the Spanish who recorded it) who came in response to the Spanish attempts to convert him and his community appeared with giant teeth, and that they warned him that the Christian missionaries were in fact tzitzimime.
While the Spanish friars he was throwing mad shade at didn't really understand what he was trying to say completely because they weren't very clear on the concept of tzitzimime, he was implying various things, including that he and his people were important enough to be visited by divine messengers (most likely, the fanged messengers were given jaguar attributes to underline their connection to divinity), that the Spanish friars were equivalent to dangerous demonic forces that threatened the equilibrium of the world and were worthy of trepidation, and that their appearance and activities were possible signals of the impending end of the Fifth World, in the same way that the tzitzimime were sometimes believed to be possible harbingers of the end of the world should their power over the dark days at the end of the year not be counteracted by appropriate rituals and sacrifices to the gods.
The idea of the incoming Christian conquerors being characterized by native priests as the malevolent tzitzimime probably illustrates better than anything else how much the ladies of the stars were viewed as a mixed bag of potentially dangerous and frightening creatures that, even when they did positive things, were still threats to the safety and continuation of the universe. Mexica myth views the tzitzimime as examples of powerful but dangerous female divinities that have cosmic significance in both positive and negative ways, and are definitely forces to be placated and feared in the Mexican religious landscape!