This is a cool premise. I like that you gave us a clear and detailed description of what the fucked up angel actually looks like. It's nice to get details like that of exactly what to imagine sometimes. Refreshing.
Totally caught me off guard with the eating its subjects part. I thought this would be a wholesome guardian angel story, haha. Also, love the detail that the subjects don't care that they will be eaten, implying the entity has significant effects on their minds/ways of thinking.
Cool!
Edit: I suggest adding a section to the containment procedures about how a new subject is introduced to the entity following the death of the old one. Like maybe some D-classes are housed nearby so one of them will be chosen, or whatever you want to write for that procedure. That seems like essential information for the people containing the entity.