Unicron is a gigantic planet of massive proportions.  Here travels the universe destroying anything that stands in his way.  From his creation, he thought that nothing could destroy him, until he learned of the Autobot Matrix of Leadership.  He needed to destroy it.  He watched the battle between Optomis Prime and Megatron.  After watching it and seeing the matrix survive, he knew he must find something to help him destroy the matrix.  Then he came upon the broken Megatron.  Unicron transformed Megatron into Galvitron.  Galvitron succeeded in getting the matrix, and tried to use it to against him.  Then Unicron showed his true power.  He transformed himself into a monstrous robot, swallowed Galvitron, and started smashing apart Cybertron.  He was destroyed when Rodamis-Prime used  the power of the Matrix.  He was completely destroyed, except for his head, which was left orbiting Cybertron.

    Then, some time after that great battle, Unicron got the spirit of Star Scream to help repair himself by attaching his head to Cybertron.  He gave Star Scream a new body, but as soon as he did, Star Scream left.  And the Autobots blew his head back into orbit.  This left Unicron's head in worse shape.