Aggregor

Aggregor is the new villain and main antagonist of Season 1 in the Ben 10 series starting with Ben 10: Ultimate Alien.

He first appeared in the Ben 10: Ultimate Alien first episode Fame. He first appeared after Ben defeated one of his captives, Bivalvin, and then captured him. He had five alien captives: Bivalvin, Galapagus, P'andor, Andreas and Ra'ad, and intended to absorb all of their powers. In the 10th episode of Ultimate Alien, he took his captives to Los Soledad to finally absorb their powers and succeeded (after ignoring the warning from Ben's former nemesis and current best friend, Kevin). The reason for absorbing his captives is to gain their powers (to become Ultimate Aggregor) in order to find 4 pieces of an artifact called the Map of Infinity in order to find a place known as the Forge of Creation.

He finds the first piece of the Map of Infinity in the episode with the same name in a temple full of traps on a planet that's half sun-like and half frozen. He finds the second piece of the map in the episode Deep. He finds the second piece on an all water planet. The third piece is found in Where The Magic Happens, where he takes the piece from the warlord from Charmcaster's home dimension (also the warlord that Aggregor took the piece of the map from is the same race as one of his alien captors, Galapagus). The fourth and final piece of the map is found in a planetoid known as the Perplexahadron in the episode with the same name from its guardian known only as The Sentinel. He finally finds the Forge of Creation in the episode with the same name and his "Ultimate Prize" was revealed to be a newly born Celestialsapien. He fights off both 10 and 16 year old Ben, Gwen and Kevin, then procides to absorb the newborn Celestialsapien. But when he was about to absorb ultimately unlimited power, he was stopped by a once again mutated Kevin, having absorbed the Ultimatrix from 16 year old Ben to stop him (even though 10 year old Ben offered him to absorb his Omnitrix) and then, after he had gotten Aggregor away from Alien X, Kevin then absorbs Aggregor's powers as well, reverting Aggregor back to his normal state and left him laying unconscious. After that, Aggregor's fate is left undetermined. It was later revealed in Night of the Living Nightmare that he is currently incarcerated.

Background
Aggregor's race is the same as Ben's current friend, Kevin (they're both members of a subspecies of humans called Osmosians). His powers are mostly the same as Kevin's- matter absorbtion and energy manipulation. Aggregor is also noted for his high intelligence, demonstrated in how he tricked the protagonists to reach his goal for him, and his genre savyness. He used his powers to activate a time transporter to absorb his captor's powers and succeeded. He's more of an intergalactic threat than Ben's ultimate nemesis, Vilgax. Like Vilgax, though, Aggregor also uses droid minions.