Keycard

Found on the final level in both golden containers and dropped off of monsters; Open up doors to rooms on the ship before the boss. There are 4 rooms in total, meaning you will need 4 keycards if you intend on opening all the doors. Collecting four keycards in one game will also unlock the Wicked Ring (you do not need to be holding them all at once, as in if you had 3, used 2 and got 1 more it would work).

Most of the time every cargo room contains one golden container.