Only thing I'd add is for a while he was the ne plus ultra of enemies; he had the highest HP in the original Monster Manual (200), the lowest AC (-8) and the highest XP in the original Dungeon Master's Guide (74,000).
Possible themes include body horror (he has two heads that hate each other, scaly flesh, and tentacles), and madness (his gaze is hypnotic). I'd say he's the most Lovecraftian of the big four, if you want to go in that direction. (Perhaps he is an avatar of Nyarlathotep, or one of the infernal lords the Nephandi serve or a Daemon Lord under Tzeentch if you want to go crossover?)