Vilgax (voiced by Steven Jay Blum) is a Chimera Sui Generis, as well as a vicious, intergalactic conqueror, and he will stop at nothing to obtain the Omnitrix and use it to build an army capable of conquering every planet in the galaxy and, alongside Kevin 11, the Forever Knights, and Dr. Animo, is the most recurring villain of the series. Vilgax serves as the main antagonist of the first season, responsible for half of Ben's enemies before appearing in person in the final episode of season one. He is, without a doubt, Ben's most powerful enemy and sworn arch-nemesis. He is one of the most feared aliens in the galaxy, having destroyed five planets and created a black hole. In fact, he is so feared, that almost no biological lifeform is willing to work for him, forcing him to use robots for his entire crew, though he is able to find mercenaries willing to work for him.
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 also cybernetically enhanced, making him more than a match for any of the Omnitrix aliens after finally emerging from the tank. Furthermore, the modifications greatly augment his physical prowess, allowing him to easily to leap above skyscrapers, survive in space for a short amount of time, and smash apart mountains, along with being able to increase his strength at will. Even before his modifications, Vilgax had uncanny resistance to harm, being able to survive a nuclear explosion at point blank range. During the Tennysons' run-in with Rojo, Vilgax establishes a psychic link with Ben (something he seems unaware of) which is how the Tennysons knew in advance about Vilgax's arrival. At the end of the first season, Vilgax tries to retrieve the Omnitrix himself, but is fought off by the Tennysons. Vilgax's ship, the Chimerian Hammer, is destroyed with him still aboard.
In "Truth", Max tells Ben and Gwen how he had a run in with Vilgax who was stealing warheads from a military base when Max and his partner showed up. In the resulting fight, Vilgax was strapped to a warhead by Max and launched into his own ship. At that time, Vilgax was only slightly larger than a human man, compared to his now colossal size (perhaps an addition to his cybernetic augmentation).
At the end of the second season, Vilgax is still alive, having survived the destruction of his ship by teleporting out just before the explosion. He is found encased in ice by Kevin 11. Discovering Kevin's connection to the Omnitrix, Vilgax decides to team up with him in another attempt to retrieve the Omnitrix, but both are locked away in the Null Void dimension by the end of the episode.
In the episode "Ben 10,000", set twenty years in the future, Vilgax eventually escaped the Null Void by stumbling across a Null Void portal being opened on another planet, but lost yet another battle to Ben 10,000. This time around, Ben didn't take any chances with Vilgax's defeat, literally tearing Vilgax to pieces. Dr. Animo resurrects Vilgax using his Transmodulator, giving him a stronger new form with enhancements such as genetically implanted weapons (such as ropes and mines in his right arm) and bio-boosters, and knowledge of all the future Ben's alien moves, 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 defeats Vilgax by freezing him in a river.
A dream version of 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 he once again attempts to retrieve the Omnitrix, having somehow escaped the Null Void. During a flashback, Tetrax reveals that Vilgax destroyed the Petrosapien homeworld, and he (Tetrax) unwittingly gave Vilgax the final element needed to carry out his plan. Vilgax refers to his anger at having been trapped in the Null Void (including showing a short nervous twitch at its mention), which has deepened his grudge against Ben to the point where he refuses to acknowledge the danger threatening the universe if the Omnitrix self-destructs (but nevertheless gains enough respect for Ben to the point of calling him by his first name instead of his last name). He confronts Ben on Tetrax's ship and attacks him, only to be shot out into space through the airlock. He is finally defeated when Azmuth, the creator of the Omnitrix, gives Ben access to a massive alien, Way Big, who hurls Vilgax into outer space.
Vilgax returns in "Goodbye and Good Riddance", yet again to steal the Omnitrix. Through the combined efforts of Ben and his father, Vilgax is caught in a natural gas explosion. It is unknown whether or not he survived, although he had to be resurrected by Animo in the episode "Ben 10,000", stating he was killed by this. However, as Ben 10,000 never explicitly stated when he "left him in pieces", it is possible that Vilgax survived and may return in Ben 10: Alien Force to cause more trouble for the Tennysons and attempt to steal the Omnitrix once more.
Vilgax is shown in his future form in Ben 10: Protector of Earth. After being released from the Null Void by Ghostfreak, Vilgax sends a small mosquito-like drone to go to the Omnitrix and extract the DNA from it. Using the DNA, Vilgax plans to use the Null Void, plus other Plumber technology, to create a vortex large enough to erase Earth from existence. To stall for time, Vilgax sends Dr. Animo, Ghostfreak, and Floraunas to distract Ben, giving him more than enough time to prepare. However, due to Ben shooting Vilgax through space via an airlock, Vilgax is warped back to the Null Void, along with his ship, and presumably the Null Void Projector. After the credits, Vilgax is shown to have survived, minus his limbs and being repaired by robots, vowing to come back.
Vilgax often responds to weak, semi-hilarous comments in a sadistic yet humorous way (such as when Ben asked "I don't suppose that means you're gonna let me go, does it?" and Vilgax responded with "hardly").