Can these two terms be used interchangeably or do these words describe two completely different things?
What correct term should I use to describe an "evil" being?
Use devil, like the King James Bible does when dealing with evil spirits.
Demons are not necessarily evil in Greek folklore, but devils always are.
>>40540284 (OP)Demons are citizens and Devils are Legion.
A Demon can be a devil or was previously a Devil, but once Legion, always Legion.
>>40540284 (OP)They're kinda synonyms but the devil is a demon and not every demon is the devil specifically. You can call demons devils if you want though. Not like anyone's keeping score.