What a neat game mechanic!
Figuring out how to connect to the clingy demons to make a usable shape is a neat challenge!
One thing that could be improved is that your combined shape is not incredibly clear, as the demons move in towards the character they're clinging to. it makes the the shape seem rounder and less clear.
The final level almost stumped me until I turned around at the very end.
(Helltaker is pretty cool. My favorite is Cerberus.)