I have a question for anyone that can remember the script better than I... didn't the knights drink from the Grail before traveling to the location where Indy found the Grail? What I mean is, as I recall, they traveled there but it wasn't until well after their normal lifespans would have ended. So, they were drinking from the Grail and extending their immortality before this seal was placed and whatnot. So, the seal idea was an afterthought? Or is this a plothole?

And who's this Fat Jedi guy using my avatar? D'oh! I'd gone nearly three years without anyone using my avatar! Jay! We need the new software so we can have unique avatars again!