Vilgax is the main villain in Ben 10 and Ben 10: Alien Force season 3. He is Grandpa Max Tennyson's arch nemisis. He wore a breathing mask in the original series, but later in Alien Force he removes it and now his mouth is visible.
Vilgax is a Chimera Sui Generis (named after the mythological monster Chimera), as well as an archenemy of Ben. He is vicious, arrogant, and dangerously relentless as he will stop at nothing to obtain the Omnitrix and use it to build an army capable of conquering every planet in the galaxy. He is one of the most feared aliens in the galaxy, having conquered 10 worlds, destroyed five others and created a black hole. In fact, he is so feared, that almost no biological life-form is willing to work for him, forcing him to use robotic Bioids and robot drones for his entire crew, though he is able to find mercenaries willing to work for him for the right price. He is also a self-proclaimed master of the Omnitrix, as he claimed that unlocking the Omnitrix's master control function was "child's play" for him, though how he gained this knowledge is unknown. In the beginning of the original series, Vilgax had no real powers, though after an accident while trying to get the Omnitrix he was cybernetically enhanced, giving him colossal size, strength and durability that made him a match for every alien Ben could access at that time. He also had the ability to temporarily increase his own muscle mass and strength. Vilgax returns in "Ben 10: Alien Force", where he gained various new abilities and equipment by draining powers from the champions of ten worlds he conquered. His new abilities include even greater strength and invulnerability, laser vision, wind breath, flight at light-speed, super-hearing, a large sword he can cover with flame or Taedenite, a red laser projector, and a gauntlet he can use to generate force fields and electric shocks. In Vengeance of Vilgax, Vilgax had defeated Ultimos and claimed his powers. He then arrived on Earth to challenge Ben Tennyson only to end up fighting Grandpa Max and the Helpers. Upon defeating them, Vilgax has Grandpa Max deliver the message to Ben. When Ben fights Vilgax, he ends up shattered as Chromastone but reforms into Diamondhead and defeats Vilgax. Due to his defeat, by intergalactic law, Vilgax had to leave Earth and never return. This did not stop him from returning to Earth a few more times. In the series finale The Final Battle, he teams up with Albedo in order to obtain the Omnitrix. When Albedo gets Ben to surrender the Omnitrix, Vilgax goes back on his deal and claims the Omnitrix to power the robots present. Albedo tries to fight him and ended up his prisoner. When Ben, Gwen, Kevin, and Grandpa Max ended up, infiltrating his ship. Ben managed to have the Omnitrix set itself to self-destruct which Vilgax thought it was a bluff. When it exploded, Vilgax's robots were useless to him and Kevin was finally reverted back to his human state. Doing the same trick to Albedo, Ben manages to obtain the Ultimatrix and fight Vilgax becoming Swampfire and transforming this form into Ultimate Swampfire. When his ship ended up submerged in the ocean, he is shown to transform into a giant squid-like creature (which he refers to as his true form) when exposed to saltwater. He then attempted to swallow Ben, but as Jetray, was able to escape the ship when it sinks further into the abyss before exploding. Though Vilgax was not shown escaping before the explosion, Ben stated that Vilgax might've escaped as he has survived worse than that. In the original series, Vilgax knew the secrets of the Omnitrix but 5 years after the original series, he cannot even use the alien selector.