Zs'Skayr

Zs'Skayr was the Ectonurite High Ecto-Lord of Anur Phaetos. He originally was stuck in the form of Ghostfreak, but after time managed to gain control over it and escape the Omnitrix, becoming one of Ben Tennyson's most dangerous enemies.

History
The original Zs'Skayr was High Ecto-Lord of Anur Phaetos. He learned about the Omnitrix when one of Vilgax's data probes crash landed and he read its data files. Not long after, he met up with Myaxx when she was scouting the planet for DNA samples and aided her in acquiring samples from the nearby planet of Flors Verdance; however, unkown by Myaxx, Zs'Skayr possessed the Florauna that was being sampled and thus Myaxx then got a Ectonurite-Flouarana hybrid sample. Zs'Skayr's personality lay dormant within the Codon Stream, as Ectonurites' personalities are tied to their DNA. The real Zs'Skayr has died sometime between then and the original series, and a genetic clone was created in full power. He now has super strength.

Original series
His mind lay within the DNA sample of Ghostfreak. He finally managed to force his way out of the Omnitrix in the episode Ghostfreaked Out. After peeling off the protective layer of skin that normally covered him, he tried to take over Ben's body in order to become whole again. After a failed attempt to possess Ben, he coerced the Circus Freak Trio to help him by taking over Thumbskull and tossing him about the room in a show of force. He was defeated when he was exposed to sunlight by Ben, causing him to burst into flames. Following this, Ben never used the form.

Zs'Skayr is the main antagonist in the third season, returning in the two-part episode The Return and 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 Zs'Skayr, whose master plan was to plunge the Earth into darkness. Zs'Skayr 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 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 Tennyson into the projector during a dogfight. In the end, Ghostfreak is killed on board a space shuttle, when Max opened the shuttle's roof and exposes him directly to the sun. Ben regains use of the form during this time, except in the Ectonurites' natural state.

Alien Force
The clone Zs'Skayr appeared in the Ben 10: Alien Force episode Ghost Town. After being removed from the Codon Stream, Zs'Skayr had apparently been imprisoned by the Galvans. Vilgax frees him on the condition that he tell him the secets of the Omnitrix and the knowledge to defeat Ben, but Zs'Skayr betrays him and takes over Vilgax's home planet of Vilgaxia, possessing the citizens and turning them into his minions that resemble his sun shielded form. Vilgax then enlists the help of Ben, Gwen Tennyson, and Kevin Levin to try to take Vilgaxia back. During the battle, Ben lets Zs'Skayr enter his body, returning the Ectonurite DNA to the Codon Stream. Ben transforms into Ghostfreak but the alien soon takes control of him and soon he tries to possess Vilgax, but is defeated by the light of Vilgax's sword and Ben changes back. He is now back in the Omnitrix. He also appears in the Vilgax Attacks Video Game as the Boss of his Home planet Anur Phetos.

Ben 10

 * Ghostfreaked Out
 * The Return
 * Be Afraid of the Dark

Ben 10: Alien Force

 * Ghost Town
 * Primus (flashback)

Ben 10: Protector of Earth
Zs'Skayr appears as the main antagonist in the Midwest. He frees Vilgax from the Null Void and teams up with him to get revenge on Ben. He also possesses Hex and tries to use his powers to bring an army of his minions from the Anur Realm, but is stopped by Ben. He leaves Hex and then attacks Ben at Chicago. Ben manages to defeat him and then sends him into the Null Void with the help of Grandpa Max.

Ben 10 Alien Force: Vilgax Attacks
It appears that Vilgax's mockery holds true, Anur Phaetos is not a planet in the technical sense but in fact a alternate dimension one of which they can't access, out of the blue Verdona arrives and states that Max sent her to help. She transports the trio to the Anur Dimension but she accidentally splits them up. To make matters worse Zs'Skayr is revealed to be "alive" and well and states that Ben was most unwise to travel into his domain. After defeating Zs'Skayr's forces Ben confronts Zs'Skayr himself and defeats him by taking full advantage of his only weakness: light. Gwen, Kevin and Ship find Ben by tracking his Mana but sadly Gwen isn't able to find Verdona. Paradox arrives and sends them back to their own dimension, he states that Max knew his transmissions were being interrupted and attempted to deceive Vilgax. While all energy cores have been destroyed Vilgax has someone under his employment making a new one from the energy in Taednite on MorOtesi, Chromastone's homeworld.

Trivia

 * Zs'Skayr is a high-lord of the Ectonurites in his home planet.
 * Zs'Skayr's name is a corruption of the word "scare."
 * In Alien Force he has normal and sharp teeth but in Vilgax Attacks his teeth are the same as from the original series.
 * In Alien Force, Zs'Skayr has his entire striped tail exposed, his claws are bone-white, he has spikes emerging from both elbows, spines jutting all down his spine, and he has fangs instead of the flat teeth in the first series.
 * In both Ben 10 and Ben 10: Alien Force no one called him Zs'Skayr - they only call him Ghostfreak.
 * In Vilgax Attacks, Zs'Skayr implies that he is undead ("Alive? Interesting word choice.","Ben 10, as I live and breathe... not!")