The Key-That-Opens-All-Locks is a key that was owned by the Hysteric Tribe and retrieved by Hiccup Horrendous Haddock III. The key has the unique ability to open all locks, how it does this remains unknown. The key is also one of the King's Lost Things.
The key is one of the King's Lost Things previously owned by Grimbeard the Ghastly.
The key opens all locks made by Hysterics, with this ability Hiccup escaped out of traps many times.
The key allows someone to become King of the Wilderwest with the other lost things.