"Battle Weary" by Mark Lup

Gradually, however, the deities of law began to suspect that the supply of demons was infinity. Weary of battle, they wished to move on to other projects, such as the creation of worlds and intelligent beings. So they made beautiful winged warriors to serve them and wield their divine magic, both in the endless war against the demons and in the worlds yet to be created. These beings, glorious in their diversity, were called angels. [...] But as the eons wore on, Asmodeus and the members of his magnificent and terrible company began to take on some of their enemies' traits, so as to fight them more effectively. Gradually, their beauty turned to ugliness, and the other deities and angels began to fear them.
- from the Fiendish Codex II: Tyrants of Hell, by Wizards of the Coast.