Board Thread:Lore Discussion/@comment-26400653-20150608022111/@comment-6881629-20150608190101

If you talk to Arngeir about your defeat of Alduin and if he did actually die he says -

"Perhaps, perhaps not. Dragons are not like normal mortal creatures, and Alduin is unique even among dragonkind. He may be permitted to return at the end of time to fulfill his destiny as the World-Eater. But that is for the Gods to decide."