Luke's Head is an NPC, a vision of Luke's head inside Darth Vader's helmet. It appeared to Luke after Luke had decapitated a psychic projection of Vader on Dagobah, and it gave Luke a cryptic warning.
The Enemy Let Slip Edit
Luke checked out a tree on Dagobah and discovered a cave underneath its roots. He went in. His sense of power increased and a dark figure emerged. Darth Vader was the figure Luke saw in the cave. Luke said aloud that Yoda must be testing him, and that Vader was just a manifestation of his own paranoia. Vader enquired if Yoda was training Luke. Luke told himself to ignore Vader and Vader would go away, addressing himself aloud as Luke. Vader identified him as Luke Satrkiller. Vader realized he must be a Jedi and declared that Vader's son would never have been killed by a mere ruffian. Vader attacked Luke with Vader's laser sword. He defended himself with his. They dueled with their laser swords. Luke yelled that Vader killed Obi-Wan. Vader gloated over this, and Luke decapitated Vader. When Vader's helmet and head fell off, part of the helmet had broken apart, revealing Luke's Head inside. Luke's Head told Luke that he (the head) was Luke's future...unless Luke defeated his (Luke's) past. Luke decided this proved the cave was a metaphysical test after all, and asked if he passed.