Devils are a race of fiendish beings that primarily reside on the Trell islands, although they have been known to appear elsewhere. The have a reputation as amoral power-hungry tyrants, as embodied by Trell’s ruler Asmodeus and his eight underlings, the Lords of Trell. Devils are known to be backstabbing and untrustworthy if it gets them ahead, and are almost always evil in the cosmic, magically detectable sense.
Devils are not to be confused with demons, which are horrific monsters that emerge sometimes from the Flux. Whereas demons are driven to destroy (chaotic evil), devils are driven to dominate (lawful evil).
See also: tieflings, race with traces of fiendish blood.
Further reading: https://dungeonsdragons.fandom.com/wiki/Devil