This item appears in a few of the levels in Beyond Dark Castle, they are, Dungeon, Swamp, and Black Forest. It is used to take out the Shovel Henchman in the respective levels.
When holding this out, jump, throw rocks, or use the Shield. Instead you can Duck, Block, or swing the Mace
The only way of dropping this weapon in Dark Castle, Beyond Dark Castle, and Color Dark Castle, is by putting it back on the wall where it came from.
This is one of the few items in the game that cannot be picked up, and taken with you.
In Return to Dark Castle they added the ability to take Weapons with you. The Mace was one of these Weapons, and now can be put away, taken with you, and used on other Enemies.
The appearance of the Mace has stayed pretty much the same between all versions of Dark Castle.