Comment by another on Tensei Shitara Slime Datta Ken - Chapter 114

@another at this point in the manga we haven't seen enough to be certain but he is one of the primordial demons, primordial black. Guy crimson is the primordial red so he's definitely extremely powerful because all of the primordial are

@another

Let me tell you a bit of backstory of primordial without spoiling anything.

They all were born in the underworld at the beginning of time. The green and blue met first and started fighting to see who can make the other one serve them. It ended with a tie so the Blue said to Green that they should team up and Green agreed. They both meet Red and Red defeats them both and takes them under him. Then those 3 meet Black and Red and Black fought for a long time and it ended up being a tie.

@another  diablo is the 2nd strongest demon 2nd to gu