- "..... SECONDED! MOTION CARRIED!"
- ―Alien X
Alien X is the Omnitrix's Sample of a Celestialsapien from the Forge of Creation.
Ben acknowledges Alien X as his most powerful alien, but concedes that transforming into him is not worth the cost.
Alien X has three personalities: Serena, the voice of love and compassion, Bellicus, the voice of rage and aggression and Ben, the voice of reason. All three speak in unison when Alien X speaks.
As Serena and Bellicus have been arguing for eternity and before they had no tiebreaker, getting them to focus on any present task is difficult at best, leaving Alien X's body trapped in place until an agreement is made.
Ben first transformed into Alien X in X=Ben+2 to stop a leaking dam. Ben soon learned that Alien X was omnipotent, but discovered a catch; in order for Alien X to do anything, 2 out his 3 personalities have to agree to do something. Bellicus and Serena only cut Ben some slack the first time because he was new. As a result, Ben spent most of the episode dealing with the headache of dealling with Bellicus and Serena. When the episode was almost over, the Omnitrix finally timed out, and Ben returned to normal.
Alien X finally made their long awaited return in So Long, and Thanks for all the Smoothies, in which Ben mistransforms into Alien X (his inteded alien was Grey Matter). to diffuse the Anihilaarg, a weapon capable of destroying an entire universe. Ben was unable to convince Bellicus and Serena to diffuse the Anihilaarg, but did manage to convince them to recreate the universe.
In Universe v. Tennyson another Celestialsapien, Starbeard, takes Ben to court for recreating the universe as Alien X, an action Celestialsapiens have a law against. Chadzmuth, Ben's lawyer, engages the Tetramand Trial of Combat, allowing Ben to fight as Four Arms to determine the outcome of the trial. Starbeard summons a Celestialsapien named Galactic Gladiator to fight for the prosecution, however, as there's no rule that says a combatant must be a tetramand, Ben uses all of his heavy hitters (Four Arms, Way Big, Humungasaur, Rath, and Gravattack).
In a last ditch attempt, Ben attempts to tranform into Brainstorm, hoping his Cerebralcrustation form is smart enough to outsmart Galactic Gladiator, but mistransforms into Alien X instead. Bellicus and Serena immediatly notice Galactic Gladiator is much more decisive than any other Celestialsapien they have ever seen, and allow Ben (at the latter's suggestion) to take full control of Alien X so they can focus on their argueing. Finally able to use Alien X to his full potential, Ben manages to render Galactic Gladiator unconsious by cloning himself numerous times, knowing his opponent's dual conciousness will disagree on which is the real thing.
See also: Custom:Ben Tennyson
Bellicus is extremely rud and impolite, Serving as the voice of Rage and Aggression.
Serena is Bellicus's exact opposite, serving as the voice of Love and Compassion.
In order for Alien X to perform any action, including simple movement and transforming back, all three of his personalities must agree to do so. This has proven to be a major weakness, as Serena and Bellicus weren't interested in dealing with present affairs and wanted to finish past ones that weren't possible to deal with without Ben.
In Ben 10,000 Returns, the said weakness was referenced by Professor Paradox, where he revealed that there is an alternate timeline where Albedo transformed into Negative Alien X and was stuck motionless for nearly a year.
Derrick J. Wyatt confirmed even Alien X can't survive on the Anur Vladias (nor can Ectonurites, even though they aren't alive in the traditional sense).
- Ironically, in many promotional images of him, Alien X is posing dramatically (like in the openings to Alien Force and Ultimate Alien), while in the actual series, most of the time, he is frozen in a standing-up position
- Before Alien X appeared, the opening of Ben 10: Alien Force showed only a silhouette of him, after he appeared, it showed his full picture.
- The only thing Bellicus and Serena actually agree on is thier mutual hatred of Professor Paradox, and only allow him to exist as long as he stays at least 500 lightyears away from any Celestialsapien.
- In Alien Force, Ben served as the tie breaker for Alien X; if two of the personalities agreed on something, the motion was carried. In Omniverse, all three personalities must agree.