Mictlan is an ancient tribal empire that practices and relies on blood magic and blood sacrifice.
Reign of Blood (Early Age)
Reign of the Lawgiver (Middle Age)
Blood and Rain (Late Age)
Mictlan is technologically not especially strong, relying heavily on obsidian rather than steel, and slings instead of bows. It is, however, magically quite dangerous, and also has some quite significant sacred troops and priests. Mictlan is a tribal society, and tribal kings have been known to recruit slave levies for the empire.
Aside from the period of the Lawgiver, Mictlan is also very strongly a blood nation; in fact, outside that era, Mictlan requires the sacrifice of human virgins to spread its dominion as mere temples and preaching do not satisfy the gods. Mictlan is one of the few nations that has this option, but for Mictlan, it's not really an option...
The prevalence of blood magic is an interesting differentiator in that blood magic is not nearly as tied to magical sites as are gem-based magic areas; it instead requires a population base and commanders spending time to collect blood slaves (preferably commanders with at least some competency in blood magic), as well as a way to deal with the resulting unrest. Blood magic is also quite concentrated in summoning.
The Mictlan player will need these summonings, for the unarmored slave levies cannot stand against charges of well-trained knights with steel weapons and battle-hardened mounts.
In the Aztec cosmology, Mictlan is the darkest, deepest part of the underworld. It is ruled by Mictlantecuhtli and Mictecacihuatl.
It is a dreadful place.
In general, the Aztec system of beliefs is a rather bloodly one. For example, it was customary for the empire's capital, Tenochtitlan, to offer daily human sacrifice to its patron deity of Huitzilopochtli. Huitzilopochtli's domains included both Sun and War, and with a number of deities demanding human sacrifice, the drive for prisoners of war ensured that both were nigh equally common. Tournaments offered some prisoners, while wars against neighboring tribes provided both -- but in the end, these same wars gave reason for the other tribes to join with Cortez in the latter's march towards Tenochtitlan.
Rain of Jaguars - summons the Ozelotl.