it was a way easier answer than retyping the scene from the book.
it's a summoned sewer monster, made from all the waste and sewage from the area. The black angel is also summoned. The summoner is off screen, since she was in a trance and not very interesting for an action scene. They fight and the angel wins.
Thanks! I am always just curious for any extra insight into an artist's work. Not being familiar with the story, I naturally wanted to know a bit more about the scene, because you've captured the action so well. Thanks for the synopsis!