Manyfaces, now corrupt by The Root. After you walk past the pillars to the end of the walk way, (seen in the Manyfaces boss arena concept: https://www.artstation.com/community/projects/Dv32Ln/edit ), you meet Manyfaces and a Root entity that controls him. Originally he was going to have Root growth coming out from him, and possibly a Root mass covering his head. It was later decided to build that creature which sits on his head in the final concept, as a massive being, so from a distance when we see the creature, it's body and tentacles are obscuring Manyfaces. Built by Tohan Kim, see his work here: https://www.artstation.com/artwork/vDBYYA