Having trouble relocating a piece of fluff about Pit Fiends.

[magicalmagicman]

I remember reading somewhere that Pit Fiends serve wizards like a minion for a long time at which point the wizard is fully corrupted and sent to hell, so the Pit Fiend wins in the long run and thats why they serve mortals.

And... I can't seem to find it again. Did i dream it? Did I imagine it? I am having some kind of a mental crisis right now. Does anyone know where this information is from? Am I delusional? It's driving me insane!

[Deophaun]

Am I delusional? It's driving me insane!
The passage you thought you read was actually an obyrith.

[Bronk]

The passage you remember matches up with the description of imps from Fiendish Codex 2. Imps are the Hell's lower ranking devil type that is involved with the corruption of mortals, and in the case of wizards they can serve as familiars.