Vilgax is a Chimera Sui Generis from a place called the Shadow Realm, as stated by the bio that comes with his action figure (later releases of the figure state his homeworld as the "Null Void Frontier"). His appearance is also very similar to Cthulhu,Cthulhu. A vicious, intergalactic conqueror, Vilgax will stop at nothing to obtain the Omnitrix and use it to build an army capable of conquering every planet in the galaxy. Vilgax serves as the main antagonist of the series in the first two seasons.

In the first season, Vilgax is confined to a regeneration tank after being severely injured by the final attack from Xylene's ship, ordering his robotic minions and cosmic bounty hunters to track down the Omnitrix for him. While his body is repaired, he is cybernetically enhanced, making him more than a match for any Omnitrix alien after finally emerging from the tank. Furthermore, the modifications greatly augment his physical powers, allowing him to easily to leap above skyscraper heights and smash apart mountains. At the end of the first season, Vilgax tries to retrieve the Omnitrix himself, but is fought off by the Tennysons. The Chimerian Hammer is destroyed with him still aboard.

In "Truth", Max tells Ben and Gwen how he had a run in with Vilgax. Vilgax was stealing warheads from a military base when Max and Phil showed up. In the resulting fight, Vilgax was strapped to a warhead by Max and launched into The Chimerian Hammer.

At the end of the second season, Vilgax is still alive, having survived the destruction of his ship but shot out into outer space somehow. He is found encased in ice by Kevin Levin and the two team up in another attempt to retrieve the Omnitrix, but both are locked away in the Null Void dimension by the end of the episode, after they use a Containment Field to remove the Omnitrix from Ben's arm.

In the episode Ben 10,000, set twenty years in the future, Vilgax eventually escaped the Null Void but lost yet another battle to Ben 10,000. This time around, Ben 10,000 didn't take any chances with Vilgax's defeat, literally tearing Vilgax to pieces. Doctor Aloysius Animo revives Vilgax using his Transmodulator and the two of them then attempt to get revenge, only to be defeated by both the present and future Bens and Gwens. Ben 10,000 ultimately defeats Vilgax by freezing him in a river. This future isn't necessarily certain, as Ben 10,000 interferes with his own past at the end of the episode, potentially altering events from there-on.

A fake/visonary Vilgax appears in the episode Perfect Day, where he appears to confront Ben. The battle lasts for a short time, and then Vilgax tries to send Ben to the Null Void. As Fourarms, Ben throws him in instead, but this proves to be an error on the part of the Forever Knights, as Vilgax was about to send Ben to the Null Void without first obtaining the Omnitrix.

Vilgax appears in Ben 10: Secret of the Omnitrix, where once again he attempts to retrieve the Omnitrix. During a flashback, Tetrax reveals that Vilgax destroyed the Petrosapien homeworld, and he unwittingly gave Vilgax the final element needed to carry out his plan. It is unknown how Vilgax escaped the Null Void, but the time spent there has deepened his grudge against Ben to the point where he refuses to acknowledge the danger threatening the universe if the Omnitrix self destructs. After a series of battles, he is defeated when Azmuth, the creator of the Omnitrix, gives Ben access to a massive alien who hurls Vilgax into outer space.

Vilgax returns in Goodbye and Good Riddance where he comes to Bellwood, but is defeated again. He took a long absence, but then reappeared in the 1 hour Ben 10: Alien Force season 3 premiere, "The Vengeance of Vilgax".

Vilgax's First Appearance
And Then There Were 10

Vilgax's Last Appearance
The Vengance of Vilgax

Vilgax Appears In These Episodes Of Ben 10
101 - "And Then There Were 10"
105 - "Hunted"
108 - "The Alliance"
113 - "Secrets"
204- "Gwen 10"
213- "Back With a Vengeance"
301- "Ben 10,000"
313- "The Visitor" (mentioned)(mentioned; his ship is seen which he is in)
401- "Perfect Day"
408- "Goodbye and Good Riddance"

In Alien Force, he appears in--
301- "The Vengeance of Vilgax"

He has also appeared in Ben 10: Protector of Earth alongside Hex, the Forever Knights and Kevin 11.