Star deities from Aztec mythology, tzitzimitls in Pathfinder are monstrous undead from the depths of space. In fact, they are creatures of such power that even many devourers and lesser nightshades have reason to fear them. You know those folks who think aliens helped build ancient civilizations, or that the Nazca Lines were meant to be seen from space? Well, imagine they were right, and then give those aliens a dash of Stephen King by way of Anthony Horowitz, and you’ve got a perfect tzitzimitl.
A tzitzimitl is a campaign-ending monster, rumored to be eclipse creators and apocalypse engines. (At the very least, they bring the deeper darkness of space with them, and they can create powerful undead like mohrgs at a rate of one per day.) Defeat one, and you’ve likely saved the world. Or if you’re throwing a truly mythic campaign, tzitzimitls are what PCs find on the worst planet possible—scions of a great darkness they must never rouse for fear that it will turn its attention to their home planet.
One should also note that in mythology tzitzimitls were female. Aside from certain drow nations, I don’t think we've had a prominent necromantic gynarchy. Maybe tzitzimitls are the perfect excuse to create one…
In a canyon so deep its bottom exists on the Plane of Earth, a nation of death-worshipping shaitans and their slaves have toiled for generations delving for the remains of a strange comet. They finally unearth the object of their search: a giant cask stuffed with the rock-covered skeletal form of a tzitzimitl.
Adventurers discover their world is hollow, stuffed with whole civilizations—some alien and futuristic, others seemingly relics of the past. But no surface-dwelling mortal was meant to set foot upon the Land of the Ochre Sun, and there are contingencies in place. In temples far away at the cardinal points of the compass, ancient undead queens sitting on far older thrones open their eyes for the first time…
When a party of adventurers ran across their first servants of the Ebon Way, they never imagined chasing the cultists would lead them to the moon and across space. But when they follow bloodmage spy Herr Kölm and his ship, The Gangrenous Heart, when they crash-land on Shroud, they discover a planet of death and nightmares where tzitzimitls march like soldiers across the blasted landscape.
—Pathfinder Bestiary 3 276
Ah, yet another Hollow World homage.
Golarion fans, James L. Sutter’s Distant Worlds has tzitzimitls being native to the lich-populated planet of Eox. Meanwhile, for my 3.5 readers: if you’re a fan Elder Evils or end-of-the-world scenarios in general, tzitzimitls fit right in to that book’s themes.
Edit: Thanks for your patience with this late post. Original entry: Only Christmas Eve could vanquish the mighty tzitzimitl. Post to follow as holidays calm down.