I’ve been away for so long. But like the winds of the summer weather, I am back and I’m even better. Nothing can break up this bond. We’re in our prime, this is our time -- Anon Guest
Legendary heroes never die. It's true. They're always shunted off into some mystical realm to rise again in their country's greatest need. Legends fade from memory. Country names change. Yet the land and the hero remain bound by that promise.
When there is the greatest need... that is up for debate. For something to be the greatest need, it has to be the most urgent disaster in all history. Which implies that this need would be occurring on the very cusp of there being no history left if someone did not intervene. There are also the heroes of legend and lore who are just... immortal.
In the time of Greatest Need... they rise. Obviously, since it's no longer the greatest need once the disaster is averted, those heroes have to a bit more than discrete if only to save face. There is, after all, always another time of greatest need.