It's complicated.
Tell me, how well do you understand modding? Can you find the "Umiertviyu" creature, its texture, and extract all of this into a separate folder, from which we can then attach it to the assassin? Or do I need to explain this too?
Well, I have an idea of how to do it. I know that I need to find the geometry, skeleton, and ID of the desired creatures and use Maya or 3ds Max to modify everything as needed, but I don't know what to do next.
If I said something wrong, please correct me.
I have basic knowledge; I know how and where to get textures, animations, effects, etc., but modifying the model itself will be more difficult.