Devils were the eventual offspring of the gods that were banished to Hell at the aftermath of the Godswar, most devils take a humanoid appearance but have lizard-like qualities like horns, wings and tails. Devils are more ordered and hierarchical than their demonic counterparts, since their conception they have been led by the first devil: Kazraiel.
The only real presence the devils have in the mortal ream is through willing servants making pacts and deals in exchange for mutually beneficial power. Generally, their attention lies with dealing with Nerat and the rebellious demons in the Eternal War.