Board Thread:Lore Discussion/@comment-6702118-20140722142914/@comment-5989209-20140723040039

Yep, and even in a weakned state, Alduin could only be killed by being forced to be mortal. He wasn't defeated by mortal weapons. He was defeated by the power of your Thu'um and Su'um. Otherwise the three heroes of old would have easily taken him as they did other dragons.