Vilgax

Vilgax is an intergalactic alien warlord and conqueror, a Chimera Sui Generis who serves as Ben's archenemy, and the most recurring villain in the ongoing franchise. He was introduced in And Then There Were 10, and has since appeared in all the seasons of the original series (however, the only season 3 episode he appeared in was Ben 10,000), as well as season 3 of Alien Force, Ultimate Alien, and Omniverse.

Vilgax was the main antagonist of the first three series (Ben 10, Alien Force, and Ultimate Alien).

Background
A vicious, galactic warlord known as "the most dangerous being in the universe". Vilgax's singular goal is to acquire the Omnitrix device, and use its technology to build an army with each troop capable of transforming into aliens. Being able to command such an unstoppable force would allow him to rule the entire universe. Though he originally chased Ben Tennyson merely because of this, and had few concerns about his fate once he had the Omnitrix. The several failures the boy caused him, as well as his imprisonment in the Null Void by his hands, his hatred for Ben grew to the point that he seems to want to kill him as much as, if not more than, getting the Omnitrix.

Vilgax is one of the most feared beings in the universe, having conquered ten planets, destroyed five others and created a black hole. He is so feared, that almost no organic life form is willing to work for him (with the exception of Psyphon), forcing him to use Drones, which are obedient machines made to serve him as his ship crew and his soldiers. However, he can find mercenaries willing to do his work for the right price.