Alien X

Alien X is a Celestialsapien from the planet Zvezda. He is an Omnitrix alien that appears in Ben 10: Alien Force.

Biography
Ben acknowledges Alien X as one of his most powerful aliens; however, he cannot afford the cost that comes with becoming the alien. Alien X is a being capable of warping reality at whim (like Anodites). He has a black body covered 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 Ben 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 first appears in X = Ben + 2, where Ben uses the form to stop a flood caused by a broken dam. After much fruitless argument, Ben eventually convinces them to transform him into one of his other nine aliens. He swears off using the form afterward, as it isn't worth the price the power comes at.

Abilities
He possesses vast reality warping powers, meaning he can make his thoughts become reality, alter matter, control living things, actions and thoughts, manipulate time, have the powers and appearance he wants, and (as confirmed by Bellicus in X = Ben + 2 when he states that Ben will never change back) stay in this form as long as he wants. He is also the strongest and most powerful alien in the Omnitrix/Ultimatrix.

Weaknesses
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 were Bellicus reopens a discussion on whether or not to save dinosaurs from extinction).

Alien Force

 * X = Ben + 2

Alien Force

 * 4" Alien X
 * Alien X (Planetary Powder)
 * Alien X (Alien Creatures set)
 * 10 cm Alien X (DNA Alien)
 * Alien X with Goop (Black version) (Creation Transporter)
 * Alien X (Defender) with Alien Bust
 * Alien X (Alien Creation Launchers)
 * Omnitrix Hero Collection with Alien X hologram
 * Alien X (Alien Rocks)

Ultimate Alien
Alien Creation Alien X and Ben

Ben 10: Alien Force (Video Game)
There was a special cheat code to unlock Alien X on the PS2, Wii, and PSP.

Ben 10 Alien Force: Vilgax Attacks
Alien X appeared in the game (only in the CGI movies/clips for X-Box 360, Wii, PS2 and PSP). He is not a playable alien.

Trivia

 * He has only been used once, in X = Ben + 2, making him the least used alien.
 * Alien X is Ben's most powerful alien, but it is also his most risky to use.
 * Before he appeared, the opening showed only a shadow of him, after he did it showed a normal image.
 * The X in his name is probably used to signify him being the 10th alien by the Roman Numeral 10. It may also relate to its mysterious nature, as X is often used in Mathematics to represent an unknown number.
 * At the beginning of the video game Vilgax Attacks, Ben starts to turn into Alien X. But Paradox tells him not to yet, but later in the game there is a cutscene where he sends Vilgax and Psyphon to the Null Void. In the DS version, after being defeated by Ben, Albedo turns into Alien X but is frozen and can not do anything. After the credits in the last level, we see a cutscene where Albedo is still frozen in Alien X.
 * Due to the fact that Alien X was on the spoiler poster, and seen on the prototype Ultimatrix alien designs, it is known that he will be in Ben 10: Ultimate Alien(he also appears unsilhouetted in the opening, indicating he might be a major alien in the new series).
 * He is also becoming a "Nano" and a NPC in Cartoon Network Universe: Fusion Fall.
 * Bellicus's name is probably based on the word bellicose, meaning aggressive or eager to fight, while Serena's name is probably based off the word serene, meaning calm or peaceful, both references to their personalities.
 * Alien X appears to be made up of space himself, hence the stars all over his body.
 * Alien X is one of the five alien forms that shares the same voice actor as it's human form. The other four being Upgrade, Benwolf, Buzzshock, and AmpFibian.
 * Alien X is one of the nine aliens to appear on a limited edition smoothey cups in Andreas’ Fault.
 * Alien X is Ben's 30th alien.
 * Through Alien Force, Kevin tries to get Ben to use Alien X even though he knows all about the risks of using him.
 * In Andreas’ Fault, a devoted fan of Ben's refers the "Alien X" smoothie as a disappointment as well as calling it "filler". This is an obvious reference to Alien X's standing in the series.
 * The three horns on Alien X's head could represent the three personalities that controls Alien X.
 * It is unknown if Albedo's version of Alien X is also associated with Serena and Belicus, or 2 different entities.
 * Alien X is may the strongest alien in the Omnitrix and the Ultimatrix.