Fiends

Fiends (devils and demons) are evil creatures that were cast from Efre Ousel by the twelve named gods. Demons were punished for their violence. Devils were punished for their cruel cunning. Any fiends are probably returning for revenge.

Unlike appendix C in the Player's Handbook (planes of existence), the Abyss and the Nine Hells are the same place. Devils and demons fight an endless war for supremacy.