Not that I'm going to argue to save this thing, but we do have other options than just killing it.
Ritual of renaming might be able to undo what happened to it. I'm still looking for the source, but I'm pretty sure that spell is part of reviving something
Unnamed. It's not the same thing, but it could be related. A little research could bridge the gap.
The actual benefit to this would be in exploring how fey work by examining an outlier state. Also, the enteral loyalty of something we saved from soul deep damnation.
I'm not saying this is the best way forward, but we might as well take a look and consider as a looting optimization. If this thing has enough power and tricks to balance the costs of the research to save it we could get more out of this than a few HD.