That is probably the key that you found just North of the Earth Elemental king (where you had to use your Pyramids to reach a golden chest). It seems there are 2 keys in the game that are called "Joker Keys".
They will open access to the Battlefield, but will not open access to a dungeon. The 2nd one of these keys can be found in Act 4 if you solve the multiple chests riddle.
I do not know if these keys were intended to work this way, but you may also notice that there are some keys missing in the game (i.e. you may find key # 1,3,4,4,5 in an act but no #6 (Act 4), or you may find keys #1,2,3,3,5,6 in an act and no #4 key). It appears that there is either a glitch in the numbering of the keys, or there is a riddle that may not be working quite correctly.
You apparently have a Joker Key.
