Vilgax is a Chimera Sui Generis, he is an intergalactic warlord and will stop at nothing to gain the Omnitrix from Ben 10. Vilgax was first seen attacking Xylene's spaceship in an attempt to retrieve the Omnitrix from her ship. He was severally injured in the attack and was confined to a regeneration tank for much of the first season and would often send drones or bounty hunters to attack Ben and retrieve the Omnitrix. When he came out of the tank his body was modified to increase his physical structure. Vilgax finds Ben and and kidnaps him and attempts to steal the Omnitrix. He is rescued by Grandpa and Gwen and battles Vilgax on top of Mout. Rushmore. At the end of the battle. Vilgax's ship self destructs while in orbit and crashes. It was later revealed that Vilgax has been to Earth before and fought Grandpa when he was a member of group that battles supernatural threats. In the second season Vilgax was found to have survived the explosion of his ship and had been shot into space. He was later found by Kevin 11. The two teamed up and tried to take the Omnitrix from Ben. The battle ended with the two being trapped in the Null Void.
Vilgax reappeared somehow escaping the Null Void in Ben 10: Secret of the Omnitrix. It was revealed that he destroyed the Petrosapien home-world home to Diamondhead's species by the petrosapien Tetrax. He was defeated when Azmuth, the creator of the Omnitrix activated a giant alien called Way Big and Ben threw him into space.
Vilgax was incinerated in a gasoline explosion caused by Ben and his dad, it is unknown if Vilgax survived, but due to his history of surviving and escaping dangerous situations he will probably seen again.
Vilgax made his Alien Force debut in season 3 with a two part episode Vengeance of Vilgax.
He had different look from original series.
Vilgax has conquered 10 worlds by defeating their biggest heroes including Ultimos and Galactic Enforcers' world. He also has absorbed all their abilities making him much more powerful than earlier. He came to earth to challenge Ben to a duel. He had some new abilities like locking laser, flight, a sword, shield and was much stronger than before, enough to defeat a fully grown Humongosaur in one hit. He ultimately lost to Diamond head.
He then teamed up with Zs'Skayr and learnt about Primus. He ended up growing giant on Primus after falling into a DNA pool only to loose once again to Way Big.
In Final Battle, he teamed up with Albedo. It was revealed that Vilgax's true habitat was water where he transforms into his true form, a giant octopus like creature. He was caught in a huge explosion caused by Ultimate Swamp Fire and was presumed dead.
Vilgax was presumed dead for a long time until Ben found out that he was posing as Diagon, an extra dimensional god. A group called Flame's Keppers were worshiping him and helping him heal.
Later he got Ascalon and freed Diagon by breaking the seal. He returned with a completely new appearance, a lot like Lucubras. He considered Diagon as his master and was helping him.
He later betrayed Diagon and absorbed him using same machine he used on 10 heroes. He ended up getting all of Diagon's power. Ben defeated him using Ascalon.
Ben 10 Omniverse
Vilgax appeared in episode "Vilgax Must Croak." In the episode, Ben and Rook was given the job of transferring Vilgax into another prison. Meanwhile Milleous gives Attea an order to kill Vilgax so Vilgaxia will surrender. Vilgax ends up saving Ben from a blast. In the end, Vilgax managed to escape.
This Vilgax looks more like his original version. He lost all the abilities he absorbed from heroes he defeated.