The Maid Of Orleans was born 600 years ago.
She's one of those remarkable characters history throws up, a peasant girl turned knightly commander and saint, a GURPS Who's Who shoo-in, an exemplary Paladin, a good candidate to be the slayer...
How would your PCs deal with an epic hero? If they're worthy of the title themselves they might be glad to meet a peer, even if their goals conflict. If they're nobodies, or even villains, it could provide a telling contrast.
Having a living legend around for any length of time risks the shadow of the GMPC, but a single guest appearance could put the PCs and their actions into perspective.