The Skeleton Key is an item you can purchase in FTK2. This guide on What Is The Skelly Key For In For The King 2 explains how and where you use the Skeleton Key as it unlocks both an in-game achievement and the Lucky Trait, as you find the secret room in The Resistance.
The Resistance is the first campaign in For The King 2. There are likely multiple ways to get this key, as For The King 2 does feature some randomly generated elements, but this method will work. You need to head into the Sewers. At the end of the first area in The Resistance you will get a choice to go into one of two locations, choose the Sewers. Then, once you are in the Sewers, there is a chance you will encounter a Goblin Merchant. This merchant sells the Skeleton Key for 100 Gold.
What Is The Skeleton Key For In For The King 2
Once you have cleared the Sewers, you will reach the second area in The Resistance. This is the area where you are speaking with the Bandit King. At the very start of this area, when the story objectives are explained, you will see Boogey’s Crypt.
One of the rooms in Boogey’s Crypt will have a door covered in a huge chain. If you do not have the Skeleton Key, you will be forced down the left path. If you have the Skeleton Key, you can use it to open the door and unlock an additional loot room.
This also gives you the achievement for finding the secret room in The Resistance.
