A predominantly Jungle world, Xibalba loosely translates to 'place of fear'. It lies only one step away from Midgard, but all known gates have been sealed tight to prevent the entities that live there from spreading.
Once ruled by the Naga, that race was supplanted by a branch of humanity who would come to excel in Life magics. They grew powerful enough to overthrow the Naga, spread their empire, and develop a government and structure based on Arcane power. In time a process known as tithing was developed in which an inferior could swear loyalty to their superior. With the oath, a portion of their arcane power would be made available.
This resulted in an Arcane arms race (see vampire) as Magi with access to unheard of power fought for dominance. Whole nations were wiped out, sometimes the victim of hostilities, sometimes due to having the life sucked out of them by out of control leaders. In time the cities, tribes, and nations were united under increasingly vast webs of tithing bonds, elevating the handful of leaders to power to rival the gods.
That level of power, held tenuously in mortal forms unsuited to carrying such potential, became unstable. The leaders, now called the Nocturne, were forced into hiding from their sun. Solar energies could potentially overload them, causing them to basically explode (and probably taking out their city with them).