This belongs in the General Monkey Island Discussion forum.
The head you speak of is the Navigator. Guybrush uses the head as a guide through Hell to find LeChuck's ship. He then, threatens to dropkick the head into the lava if he does not give up his eyeball necklace, which kept Guybrush invisible from the ghosts on LeChuck's ship.
And by the way, welcome to LucasForums!