Zs'Skayr

Zs'Skayr was one of Ben's forms, freed from the Omnitrix. A copy of his evil personality was trapped within the Omnitrix when his DNA was sampled. He finally manages to force his way out of the Omnitrix in "Ghostfreaked Out" After peeling off the protective layer of skin that normally covers him, he tries to take over Ben's body in order to become whole again. After a failed attempt to possess Ben, he coerces Acid Breath, Thumbskull, and Frightwig to help him by taking over Thumbskull and tossing him about the room in a show of force. He is defeated when he is exposed to sunlight by Ben at the end of the episode, causing him to burst into flames. Following this, Ben never uses the form.

Ghostfreak is the main antagonist in the third season, returning in the two-part episode "The Return/Be Afraid of the Dark" where he is revived by an alien scientist named Dr. Vicktor. Gathering a group of horror-themed aliens as minions, Vicktor enacted a plan to resurrect Ghostfreak, whose master plan was to plunge the Earth into darkness. Ghostfreak himself is still intent on gaining control of the Omnitrix. Using corrodium retrieved by the mummy alien in "Under Wraps" they set up a Corrodium beam beam that will block out light from the sun, allowing Ghostfreak to exist in eternal darkness. Ghostfreak succeeds in doing so, allowing him to reach his full potential, but only for a while before Doctor Vicktor accidentally rammed Max into the Corrodium Projector during a dogfight. In the end, Ghostfreak is killed on board a space shuttle, when Max opens the shuttle's roof and exposes him directly to the sun. Ben regains use of the form during this time, except in Ghostfreak's natural state.