From MIwiki
LeChuck, trying to hold on to the voodoo cannonball
is a magical item that
LeChuck intended to use to blow down the walls of
Elaine's island fortress at the beginning of
The Curse of Monkey Island. A cannonball flaming with green Voodoo magic it only manages to prove its destructiveness when
Guybrush, trapped beneath deck, fires a cannon after having cut its restraint-rope. LeChuck drops the cannonball which blows up, destroys him (temporarily) and causes the
Death Starfish to capsize.
