Who was Dimentio

I think he's either the son of the magician who created the first pixls or the writer of the Dark Prognosticus, maybe both.

I read somewhere (I think it was a fanfiction) that he was actually Count Bleck's father. I used to believe that until I looked more at the actual backstories of SPM and then I find it much less believable.

A piece of paper

It is hinted that he is the author, no doubt about that part.

I wish he had a scene with Shadoo. It would have been cool if they recognized each other.

Oh, that's simple. He was Phone.
...
In all seriousness, Dimentio definitely had something to do with the Dark Prognosticus, whether as the number one guess of writer or something else. He knew what was written in it, despite Bleck never letting it out of his sight on-screen.