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.
One of the kings lost things previously owned by Grimbeard the Ghastly
Opens all locks, with this ability Hiccup escaped out of traps many times.