And that's it. Thou Shalt Not Get Caught. The issue at hand is to decide what exactly Arik is. Obviously, he is an Entropic Immortal. Personally, I consider it doubtful - the style of power seems too different.
Demons are about grey hopelessness, grim destruction, decadence and corruption.
While there isn't much to go on in B3, he seems to be more interested in intelligent corruption than the bestial image the demonic entropics present. But the flamboyant flair for grand-scale destruction that seems present in B3 seems to be 'more' than any devil would do. Above all, the Devil works in devious ways. Filling a valley with orcs and undead, surrounding a palace in a ruby-red forcefield, and turning every man, woman and child in it to stone could be called many things, but subtle isn't one of them. Given the large number of eyes his Immortal form uses, it's easy to suggest that he was a beholder or beholderkin.