I happened upon a skeleton chest that was locked anyway to unlock just came back after a bit of break first time i've ran into this.
Skeleton chest that's locked?
So all chests in this game come locked and you'll need to turn it into an outpost to gets the loot inside! By that I mean just gold and Gold Hoarder XP. The only exception are the collectors chest which can be opened immediately but theres nothing inside. These chests are used to transport other items like skulls and trinkets.
Every outpost will have a representative of the Gold Hoarders faction in which you'll turn in chests and trinkets to (goblets, relics, etc.). These Gold Hoarder representatives are always in a green tent near the other vendors.
There is a locked Skeleton Chest that is part of Tall Tale #2 (The Cursed Rogue) and requires a Skeleton Key to open. I'm not sure where you found it, but it does not just spawn randomly. This had to be left somewhere by another player that abandoned the Tall Tale they were working on.
