A devil is a four dimensional creature original to the Infernal Plane, they are born when a damned spirit collects enough soul to enlarge themselves over the third dimension, as such, devils do not have a body and so need to possess one when summoned.
Celestial devils
If a blessed spirit happens to collect soul like a damned one would, they’d eventually turn into a devil themselves, no different in features than an infernal devil, this has never happened before, as most blessed souls have no wish for power.
Mixing with mortals
Devils are the original carriers of the tiefling gene, offspring of a mortal and a devil will not be a tiefling themselves but rather the race of the mortal parent, and all children will be carriers of the gene.
It’s of note that offspring of a devil and a mortal is not a Demigod as one might think.
In culture
Devils are justifiably considered the root of all evil by most mortals, it is taboo to summon and/or make a deal with one.
Spread
Devils are almost exclusively found in the infernal plane.
Notable devils
- Emperor Jason Seraph, post mortem
- Ser Ghastly of Grenalt, post mortem