Heroes have a tendency to be foretold by prophecy in fantasy stories. It sometimes seems to be few and far between that a hero worth his salt won’t have at least a single prophecy foretelling his coming or his return. But those prophecies can sometimes be centuries old, and we all know how accurately things get passed down through the ages, and that’s without considering the problems which arise when you’re translating from one language to another.
It’s possible that something like the internet would ensure that the prophecy in its original form remains intact (if the prophecy wasn’t made until around the time that such a thing existed, anyways), but how many alternate versions would pop up, too?
So let’s have this mighty hero, who does xir heroic stuff and then departs from the land in some fashion. It doesn’t matter if xe goes into the blessed isle, or enters an enchanted sleep, or ascends to the gods. Before xe's gone, though, xe promises his eventual return and gives signs by which xe might be known.
Fast word centuries later, and xe's come back, maybe just in time to save the world but definitely to do something important. Problem is, nobody remembers accurately the signs xe gave, and there isn’t even just a single corrupt account, but several. And some don’t even think that xe'll be coming back at all, or existed in the first place.