I noticed that Demise's skin looks a LOT like the Imprisoned's scales. Demise. The Imprisoned. I have no idea if this is confirmed as I've been avoiding major spoilers, but it makes sense. Please tell me if this was confirmed, and if not, please comment.
It is confirmed that you fight The Imprisoned first as an earlier form of Demise. He is later revived to his more demonic/humanoid form as the final boss. So yes, it is confirmed.
Please put a spoiler tag on something like this next time... I know it's supposed to be theory but it's frustrating when trying to avoid spoilers and especially when some are included in a thread without at least a spoiler tag.