The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
An image search turned it up as a free wallpaper download of a "fire demon," artist unknown. The wiki shaves off the website in the bottom right corner.
His avatar is actually more like a sphere of annihilation, and in a 3E mega adventure, one of his servants taking his form looks like this: