Creatures of Tanrilar Dunyasi
Since The Cataclysm, all mortal creatures of Tanrilarin Dunyasi become undead the following night at midnight. They generally simply become zombies, which eventually decay into skeletons, but mortals which have underwent particularly significant events may turn into a different form of undead depending on the nature of those events. Similarly, those infected by many types of undead will instead turn into that type of undead. Once an undead creature has been destroyed, it will not return unless brought back by necromancy as would normally be the case.
Outsiders, such as devils, demons, angels, etc… are all immaterial spirits that are attracted to mortals based on their emotions. They feed on these emotions, and if a mortal consistently deals with a particular emotion to a very high level, they may be able to possess the body of the mortal, exerting strong influence on it as fits their type. Given enough time, an outsider in possession of a mortal will be able to harvest enough energy to physically manifest, at which point it will split off in a material form, with effects on the possessed varying depending on situation and type of outsider. Once split off, it will be able to act as a completely independent creature, generally of great potency, until its physical form is destroyed (or convinced to disband) at which point it will return to its spirit form to once again attempt to feed.
Elemental outsiders function in much the same way, except that they feed off particularly significant material bodies which have come to hold potency in ways that is not truly understood by scholars.