Many people think this card is totally broken. Is it really? Let's analyze it.

Costs

• 5 XP: Very expensive, but reasonable for such a powerful effect. We're lucky it's not exceptional.
• 3 Ressources: Reasonable price, which most investigators should be able to pay. Only Dark Horse decks may find it too expensive.
• Relic Slot: Shares the slots with some other powerful cards, like Elder Sign Amulet, Holy Rosary or St. Hubert's Key. Most of these cards are mystic, so the slot is ok. Relic Hunter is also an option.

Requirements

In order to get the most out of this card, you need:

• In play as early as possible
• A reliable source of horror
• horror soaks to protect it

Getting the key into play

Only way to draw this card early is hard mulligan and seeker cards:

• Preposterous Sketches, but only for the person playing it. All others can be played on other people.
• Eureka!
• No Stone Unturned 0, No Stone Unturned 5
• Cryptic Research
• Eidetic Memory when copying one of these cards

Horror Sources

• Forbidden Knowledge
• Scrying 3
• Shrivelling, but very situational

• Painkillers, but only if there's damage on your investigator

• Most of these are cards. Other than these, enemies are the most reliable source of horror. But they often deal damage as well, so you need damage soaks (Bulletproof Vest) and/or healing (Strange Solution).
• To play it safe, stop taking horror intentionally after the second, so it it's bonus is only at +2. Then it won't leave play when you take another horror wihtout a soak.

Horror Soaks

• Neutral: Elder Sign Amulet is the best neutral soak, but it's also a relic and requires Relic Hunter in addition. Calling in Favors can be used to re-play an ally, healing him in the process.
• Guardian: Brother Xavier or any ally. Heal them with Inspiring Presence
• Seeker: Has Lot's of disposable allies like Art Student or Laboratory Assistant
• Mystic: Holy Rosary or any ally.
• Rogue: Several expensive allies. If possible use a different class or neutral cards.
• Survivor: After Cherished Keepsake, there's Peter Sylvestre 0 and Peter Sylvestre 2, the best horror soaks in the game.

Other things to note:

• When taking more than 1 horror, you can place any of them on Key of Ys
• This card can replace other boosters, like Spirit Athame or Dark Horse
• Direct horror can't be redirected, even when this card is in play.
• You can't use Agnes Bakers ability while this card is play, unless it's direct horror

Summary

This card is definately powerful, but uesless if you can't put any horror on it or it's destroyed. So i wouldn't put my first 5 XP into this card, unless your deck (or your party) is built around it.

A strong contender for most unplayable card in the game right now, at least if we're talking Hard/Expert. The upcoming Finn Edwards (who will have 4 Combat) may be able to use this card, especially since he will be very good at exhausting enemies first (which turns off Retaliate). But for Skids or Jenny, this is 100% unusable on anything but Easy, and even there it's unnecessarily dangerous and I would not recommend it. Obviously, Sefina can't even consider it.

(Update: Finn actually has 3 Combat. So I think this is pretty bad even for him.)

Although consistent +1 damage is one of the most desired traits in a weapon, the fact is that most enemies in this game don't have Retaliate. Given that Knuckleduster lacks a to-hit bonus, and that Rogues have low strength, using this card means subjecting yourself to a ton of unnecessary damage. You can evade and exhaust the enemy first to avoid this, but I really don't recommend this. Evading enemies and then fighting them is incredibly wasteful for any investigator not named Finn. Either just kill them or evade them, but don't waste actions doing both. Rogues have plenty of other weapons to pick over this.

This card continues a very weird trend in the design of Rogues where the class gets cards that don't remotely fit the investigators. Rogue weapons like Knuckleduster, Switchblade, and .41 Derringer all seem designed to work for investigators with really high strength. Except, you know, none of the Rogues actually have that, so Rogues are terrible with their own weapons. (In contrast, Survivor weapons like the Fire Axe and Baseball Bat are much more sensibly designed to work for low-strength investigators, so Survivors are actually good with them.)

Anyway, if you asked me to name the least useful card in the game right now, I think I might pick this one. Although, come to think of it, at least it doesn't cost 4 XP like the Springfield M1903 does. So maybe it's the only second-least useful card in the game. That's something.

This card can target any card in a players deck, including weaknesses. This aspects usefulness depends on the weakness in question:

• Some weakness do nothing, when drawn early: Abandoned and Alone, Shell Shock, Crisis of Identity
• Some are trivial when the party is spread out: Searching for Izzie, Smite the Wicked
• Some weaknesses need time before they can be solved, so it's important to draw them before the mid game: Hospital Debts, Cover Up, Baron Samedi, Angered Spirits
• Others require some preperation: Any enemy weakness, Overzealous, The King in Yellow, Final Rhapsody
• Some should be killed over and over for story reasons: spoiler

Other combos:

• Get powerful signature cards into play early: Green Man Medallion, Spirit-Speaker, Split the Angle
• Save deck space by including only 1 copy of unique or limited assets, like Lone Wolf, Dark Horse or Leo De Luca
• Can be copied wtih Eidetic Memory
• Gets better with more players, as the target card pool gets bigger and with that, solutions to challenges
• Can be played in any player window, including skill tests during mythos phase. This may save a player, as he could search his deck for the appropriate skill icons, like Guts against Rotting Remains

At the time of writing this had only been out a few days - and I've used it in 3 games...

This card is quietly awesome - if you, or a fellow investigator, have another card that you really need.

It's not flashy. It isn't fancy. There's no cool combos I can see. Letting someone draw any card from a deck doesn't seem that spectacular. But...

• your Guardian needs his Shotgun - fixed.
• Minh needs her Analytical Mind - no problem.
• Your mystic needs a Shrivelling right now, okay ... done.

No fuss, no worries about weaknesses or where it is in the deck, just for 2 resources take any card. It doesn't even cost you an action.

Okay, that is spectacular.

The down side is that 5 XP is a lot of XP. It's up against flashier cards, like the The Red-Gloved Man, Deciphered Reality, Strange Solution. And it's only as good as the card you need to grab, so it probably isn't a good pick for early in a campaign. However, later on, when you've a few high-value cards in the deck - this one is a solid choice.

(That said, for Minh Thi Phan, Analytical Mind is so central to her that I could imagine taking this early for her)

A really solid multiplayer pick.

Solo, it loses too much benefit to be worth it's XP, and Seekers have plenty of access to filtered draw through Old Book of Lore, Eureka!, and No Stone Unturned anyway.

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.