Alien X in the Original Series
Alien X is a Celestialsapien from The Forge of Creation and is in the series Ben 10: Alien Force and Ben 10: Ultimate Alien.
Backstory
Ben acknowledges Alien X as his strongest and most powerful alien; however, he cannot afford the cost that comes with becoming the alien. Alien X is capable of warping reality at will. He has a black body filled with tiny white stars, white hands, green eyes, and three horns protruding from his head. He has three distinct 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. In order for Alien X to perform any action, including but not limited to, simple movement and transforming back into Ben, two of the three personalities must agree to do so. As both Serena and Bellicus have been arguing for an eternity, and before they had no tiebreaker, getting them to focus on any present task is difficult at best, leaving the body of Alien X frozen in place while Ben attempts to convince them to act.
Alien X standing
Power and Abilities
Alien X has shown to be practically omnipotent, able to warp reality and time/space itself, becoming nearly invincible. His reality warping is so powerful, that his mere thoughts can become real. Because he is Ben's most powerful alien (or so Ben thinks, as Dwayne McDuffie states that he is not the most powerful alien in the Ultimatrix) and carries the most risk. He has only demonstrated his powers twice.
Alien X
Weakness
In order for Alien X to perform any action, including but not limited to, simple movement and transforming back into Ben, at least two of the three personalities (Serena, Bellicus, or Ben) must agree to do so. This reveals to be a major weakness, as the personalities initially weren't even interested in dealing with present affairs and wanted to finish some past ones that weren't possible to deal with without Ben (with a comical scene where Bellicus reopens a discussion on whether or not to save the dinosaurs from extinction). In Ben 10,000 Returns, the said weakness was referenced by Professor Paradox, where he revealed that there is an alternate timeline where the Albedo of that universe transformed into Alien X and was stuck in the form for nearly a year.
On another note, despite his omnipotence, he is vulnerable to having his powers absorbed, as Ben and Azmuth respectively have implied that Darkstar and Osmosians can steal his omnipotent cosmic abilities and powers for themselves, though Osmosians will gain all the Celestialsapien's abilities whereas Darkstar will just grow more powerful not gaining it's abilities.
Evolution
Evolved Form: Ultimate Alien X
When Alien X is evolved into Ultimate Alien X his two personalities, Serena and Bellicus, are removed from his conscious and he is made completely invincible. His appearance changes are that he becomes more taller and muscular looking and his color is inverted.
Ultimate Alien X