Temporarily Bound Pit Fiend
Many stories abound of this particular pit fiend, who over the past several hundred years has been commonly summoned to the Prime Material Plane by arcane binding spells. He is usually seen to be torturing a fiery female figure in the palm of his hand, flaying pieces of her with his claws with every sign of glee as his actions unsettle those that seek the knowledge he holds.
PLAYER EDIT: Mannoroth is actually a Greater Demon, and thus can only be killed in The Abyss. If destroyed elsewhere, his spirit will travel there and re-form. Greater Demons can posses an amulet like a phylactery that will allow his spirit to linger on, even if killed in The Abyss.