The Skeleton Key is an accessory in Spelunky 2. It can be found in shops or by making sacrifices to Kali, and is also a relatively rare drop when a skeleton block is destroyed or when a skeleton is killed. It can also be obtained by killing Sparrow.
The Skeleton Key is capable of unlocking any locked gate, similar to the capabilities of a Metal Key. Unlike Metal Keys, the Skeleton Key is a passive item which does not need to be held and has unlimited uses. Despite what the item's description in the Journal seems to imply, it is not capable of unlocking the chest containing the Udjat Eye.