"The tool of thieves. It opens any lock without breaking."
The Skeleton Key is capable of unlocking any locked door without breaking, meaning it can be used multiple times. However, despite what the item's description in the Journal seems to imply, it is not capable of unlocking the chest containing the Udjat Eye. 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.