"Nostradamus Rule:" All legends are 100% accurate. All rumors are entirely factual. All prophecies will come true, and not just someday but almost immediately.
There is a very, very good reason for this: the depiction of false myths is boring. It is the fantasy gaming equivalent to opening Al Capone's vault. There's no point in spreading a myth out for the party to investigate if there's no reality to the myth ... and besides, myths are interesting frameworks upon which to hinge games. They make fine mcguffins, theatrical climaxes and ethical conundrums. Why, we wouldn't even have these myths if its wasn't that the fascination about them endures.
What is sad and pathetic about the use of myths in games is the relatively short list that exists in the ordinary imagination. Not only because the DM can't come up with something new and interesting by reading a few books, but more to the point that players aren't likely to get fascinated with a myth of which they've never heard. If you haven't heard the myth before, it hardly carries much sentiment and romance ... in fact, the DM might as well have made it up in his or her own head, for all the oomph the myth has. Thus it happens that if you're going to depend on myths for your game, to get the oomph, you're stuck with very, very few choices.
And its worse if, as a DM, even you can't get interested in your own imagination.
So what does that give us? Well, everyone has seen Indiana Jones, Stargate, Buffy, Sanctuary, True Blood, etcetera, etcetera ... so you're safe if you mine those sources for your world. But there's a pretty big uggh factor if your players have considerable gaming experience: "Again, Bob? Another sword in the stone? That makes three this year, doesn't it?" And there I am, stealing the example from the TV tropes page just to hammer the point home.
If your players have managed around five years of steady game play, I promise you they've seen virtually everything ... that is, the habitually reworked list that television and the movies reserves for their Monster of the Week formula. The emotional juice you hope to wring from that dessicated and worm-eaten harvest is pretty much depleted. Expect your players to repeat the formula in their heads as you outline the tale at the table. Yeah, yeah, a vampire is loose in the neighborhood, there's a chick asleep in the castle, merlin left this hear before moving back in time, the spear of destiny is buried in zombie Hitler's tomb, Achilles is assaulting Troy, the sphinx has three questions and Nefertiri is ready to spread 'em because, well, she just wants to.
A great many DMs invoke these things from their pseudo-mastubatory appeal, like a cosplayer who can't wait to dress like Wonder Woman or the Green Gunker (sorry, there's about fifteen 'Green' choices, I just went with the word that defines my emotional feel about all of them). "Wow," thinks the DM; "won't it be really, really cool to have the Loch Ness monster in my campaign!" Or characters from The Planet of the Apes or the Queen from Snow White (who really ought to be dead) or whatever the hell else turns their crank. Sometimes, you can watch your DM get a little obsessed with an invocation like this ... it's creepy.
Of course, if you can figure out a way to introduce the monster/item/event with an unexpected originality, you will probably be able to pull your party out of its malaise ... but chances are that 'new' is going to be a lot closer to the disastrous discontinuity of Van Helsing rather than effect you desire. But you're welcome to prove me wrong.
I think it needs to be said that, in reality, you don't really need to invoke myth in order to give your campaign verve. It's a crutch, anyway. It's what you employ for lack of a good idea. That's why television leans so hard on it.
But the very young and unjaded will still like it, because they're young and unjaded. But if they're that young, they better be your own children.