Skeleton Keys are old style keys used open many things.
Another Skeleton Key was a key kept by the Keyholder. Alexander was able to get the key by making the skeletons dance. It could unlock many things such as the Vizier's trunk, and the doors to the Castle of the Crown's dungeon.
Behind the scenesEdit
Its interesting to note that Alexander sees no place for the key to be used on the Underworld Entrance. Its unclear what purpose the key is actually used for.
The key in KQ6 is known as the 'Skeleton Key' in KQ6 Hintbook.