As far as i remember from early and old belial-universe stories, there are special demon-traps called demon-bells.

They are powerful magical artefacts, which can imprison even mighty avatars. I guess a demon hunting party has nearly no chance to kill a greater avatar, but if they have a demon bell, they might be able to trap the avatar into it. I'm not sure in which story i had read that, and there is a small chance that i'm completely confused about it.

Well, if my memory is correct, then i have a neat (IMHO) idea for a story: A legendary demon hunting party  was able to trap a avatar of karma, but had to pay a horrible price.
Somehow the demon was able to damage the original bell, and the sorceress had to sacrifice her own body to merge with the bell and stabilize it. The result was that the avatar was actually trapped in  her body.

Of course she doesn't look normal after that, but she is still able to think and to move (i have several ideas for the form, but i wanna get a general OK for my idea, before i finalize it)

Unfortunately the party learns later that the improvised trap isn't stable and demonic "essence" is very slowly leaking out of the sorceress, transforming her further and even worse taking slowly over her body. It begins with penis tentacles that she can't control and in the end she probably transforms into a immobile pile of genitals or maybe something like that.