This item only appears in one level in Dark Castle, and that level is Trouble 3, it is used to take out the Whip Henchman to get to the Key.
When holding this out, you cannot turn round, jump, throw rocks, duck, or use the Shield.
The only way of dropping this weapon in Dark Castle, Beyond Dark Castle, and Color Dark Castle, is by hitting the Whip Henchman
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 Morning Star was one of these weapons, and now can be put away, taken with you, and used on other Enemies.
The appearance of the Morning Star has stayed pretty much the same between all versions of Dark Castle.