The Ahn are the gods. There are three of them Rheina, god of firmament, Nihko, god of death, and Mikayal, god of soul. The Ahn'an, Ahn'tep, and Daem'Ahn are each races created by the gods. The Ahn'an are the fae and are composed of Rheina and Mikayal. The Ahn'tep are the Eihelvanan, Darwaven, Argonts, and humans. The Ahn'tep are composed of all three gods so unlike the fae the Ahn'tep's soul can pass into the afterlife (and apparently come back from the afterlife) while the fae's soul will pass into Ahgre'an when the fae dies losing itself and joining with the rest of the fae. The Daem'Ahn are demons and are composed of Rheina and Nihko. They have no soul and only want to spread chaos. The Ahn have decreed that the main realm (I don't know what to call it we haven't been given a name) is now the realm of the Ahn'tep. The decree was handed down thousands of years ago. The Ahn'an can live in the human realm as long as they don't interact/kill humans (the rules for this are not clear Bilior has said he can't even kill humans that demons are possesing since they are still humans but they don't have a soul). The Daem'Ahn can enter the human realm from H'khanjnak (the demon realm) by possessing a human, they may or may not be able to physically enter the human realm. After the third book we have seen three people possessed by demons and can safely assume that Caeden is also possessed by a demon and Bilior hints that many in Ashai are also possessed by demons.