This weapon is one of the weapons available in the Armory in Return to Dark Castle. It is a hammer (near a mallet in size) and is used to take out any henchmen. It is as powerful as the Mace and Shovel.
In Return to Dark Castle, the hammer can be picked up and put into your inventory. To drop it, you can press E + Q.
Looks pretty much like a mallet in Return to Dark Castle.