The Tzitzimimeh in Aztec mythology are a group of skeletal, rattlesnake-endowed, Jolly-Roger wearing, fertility-inducing, star goddess-demon things of chaos that periodically attack the sun and come down to Earth to eat people.
That. . . That's really it.
The average Tzitzimitl (Tzitzimimeh is the plural. Two typically terrifying Tzitzimitls together turn to Tzitzimimeh) is a skeletal figure of a woman -skeletal as in 'fleshless' rather than 'skinny'- in skirts covered in skull and crossbones designs, and have penii made of rattlesnakes.
As far as the terrifying acid trip that is the Aztec pantheon goes, these are actually some of the more reasonable deities, so bear with me. They are usually considered to be star goddesses with a side of fertility. Specifically, they are associated to the stars seen near the sun during a solar eclipse. The reason there are solar eclipses at all is because the Tzitzimitl are attacking the sun. Obviously.
During times of cosmic instability (the sun being blotted out and possibly dying forever, the changing of the year and the calender, etc.) the Tzitzimitl descend from on high and ravage the country, eating people stupid enough to be wandering out and about when the sun is up there fighting for its life, you inconsiderate prick! Don't you care at all about the sun? Jeez!