Alduin was defeated not only in the mortal plane, but Sovngarde as well, by the Dovakiin himself. But the firstborn of Akatosh is immortal. He may be defeated, but not destroyed. If he cannot take the souls of Skyrim, he'll look elsewhere.
A verifyable madman! Indeed, I may or may not be. I am, however, a brony, which is why I'm here. Obviously. If you cannot see that, you must need special glasses. No offense to people with glasses.