
I think this is the best card Arkham Horror: LCG has released since Higher Education. Key of Ys is absurdly broken.
For best results, try it with Duke and Peter Sylvestre as Pete. Those two allies will keep the Key in play more or less indefinitely, and you can enjoy what is effectively an 8/7/7/7 stat line. That is totally gamebreaking.
Or try it as Yorick. He can also use the Peter Sylvestre combo, and if the Key somehow does get discarded, Yorick can just play it back! (More often than not 10 discarded cards actually helps him, too.)
Survivors are best poised to take advantage of the Key, but really, anyone can put this to great use.
Yes, it's 5 XP, and you need other sanity-soaking assets in play for it to really work, but the benefits it grants are so absurdly strong that these drawbacks don't come close to balancing it. No matter who you are, the Key makes you an all-around dominant investigator. This card makes Wendy fight like Zoey and Skids resist treacheries like Agnes. What is more, it augments every investigator's strengths, to the point that even the Expert chaos bag stops being that scary. No other card in the game comes close to that.
The "discard the top 10 cards of your deck" proviso is totally irrelevant unless the scenario features Beyond the Veil. Even then, you might not draw Beyond the Veil, and if you do, you can just be careful about keeping the Key in play. That's not a major concern at all.