• Q: How does The Skeleton Key interact with Obscuring Fog or Lantern? A: If an effect "sets" the value of a statistic, that overrides all other modifiers. In other words, if a 4 shroud location has Obscuring Fog and The Skeleton Key attached to it, its shroud would still be 1, no matter what, because The Skeleton Key sets its value to 1. It is worth noting that this answer would be different if it set its "base" value – the "base" value of something is the value before modifiers are applied, so that would allow it to be modified afterward. But here Skeleton Key sets its value to 1, AKA the total value after all modifications.

• Erratum: This card's ability should read: "If The Skeleton Key is in your play area, attach it to your location…" - FAQ, v.1.41, October 2018

TL;DR: Think of the key as a late-mission MP support Event, not an Asset. It competes with no slots, and you don't have to play it until/unless it's needed. If 3R and 1 action to trivialize an entire 3+ Shroud 8+ Clue location for EVERYBODY in your party seems worth it, then it doesn't matter when you draw your one copy- you'll always be happy to see it.

+++

I recently completed a 3P Carcosa as Tony Morgan; the key here was a late upgrade in the "eh why not" vein after getting all his other needed upgrades. It ended up being a relief to see on each remaining mission.

Left me wondering why it never really got much in the way of reviews, but I think I can see why- it looks deceptively cumbersome. 3R to play, 2 actions to place & recover, only 1 per deck, Lockpicks exist, etc. But here's the thing: in 3-4 player MP, all of these become unimportant... a single use of the key will pay for itself.

• Fast. No need to play it til you need it, and ultimately costs only 1 action to place it at a destination.
• NO SLOT. Did you notice that? Some assume the key is an Accessory or Hand item at a glance, and thus in competition.
• Item. You can tutor it with Backpack, which many Rogues enjoy running.
• You likely want it late. High-shroud spots often reveal near end of mission. This reduces the 1-per-deck risk.
• Scales hugely in MP.

That last one's the biggy. It's rare that a campaign doesn't have at least one location with 3+ shroud and 2+/Investigator clues. In SP you can clear that with an event or skills. In 4P though, the Skeleton Key becomes an 8-charge Flashlight that your whole party can use in addition to any other investigation skills they have.

If you can get the key to work on 1 hard room in a mission in 3-4 player MP, it's a terrific and reliable value. The uninstall/reinstall effect is neat, but distracts from the fact that it should pay for itself with a single use.

(FUN FACT: Preston & Dexter & Sefina aside, everyone who can take the Key has 3+ base INT, and is thus themselves now investigating at a minimum of +2 after using it.)

(edited to add Sefina to sub-3 INT investigators, thanks to all who noted this in comments!)

• Can other investigators use the keys ability to move it to their play area when it's attached and later attach it themselves?
• If not, what happens when this card is moved to another player's play area with Teamwork? Is the new controller the only one you can attach and detach it? Or is the owner the only one who could detach it, after the new controller attached it?

Skeleton Key might be a strong "maybe" purchase for Rogues and off-Rogues in multiplayer, but I'm a little surprised no one has mentioned how excellent it is for Ursula Downs.

• Seekers love this card to begin with, as it makes their job easier, so being able to include it in a deck is fantastic.
• Dr. Elli Horowitz and the rest of the deck searching powers of the card pool mitigate the Exceptional restriction.
• Ursula's action compression allows her to budget the needed for moving the key.
• Her upgrade pool is pretty limited, so not too much competition beyond the big-ticket upgrades.