Alien X

Alien X is the Codon Stream's DNA sample of a Celestialsapien from The Forge of Creation.

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 personalities: Serena, the voice of love and compassion, Bellicus, the voice of rage and aggression and Ben, the voice of reason. All three speak together 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 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.

Powers and Abilities
Alien X has shown to be 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 Ben thinks, as Dwayne states that he is not the most powerful alien in the Codon Stream, but carries the most risk.

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 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 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 steal his omnipotent cosmic abilities and powers for themselves.

Alien Force and Ultimate Alien

 * Alien X first appears in X = Ben + 2, where Alien X stops a flood caused by a broken dam. After much fruitless argument, Ben convinces Serena and Bellicus to transform him into another alien (mostly by insulting them). He swears off using Alien X afterward, as it is not worth the price even when there is immeasurably strong and powerful universal power that comes with the form.
 * In Darkstar Rising, Kevin suggests to use Alien X against Darkstar. Ben declines as there would be no stopping him if he had absorbed Alien X.
 * Alien X returned to Ultimate Alien in The Forge Of Creation, Ben halfheartedly used Alien X in hopes of locating the Forge of Creation in his last-ditch effort to stop Ultimate Aggregor from obtaining the universe's greatest power. It was revealed that Ben and Kevin hacked the Ultimatrix offscreen and devised a special lock system to prevent bad guys, like Albedo, from transforming to Alien X. With this, using Alien X now requires two keys to be inserted to unlock Alien X. While Alien X is being hidden from the playlist, the keys that which unlocks him are being kept by Gwen and Kevin, who both own one of the keys. After transforming, Bellicus and Serena start debating about other things as usual. Suddenly Paradox appears, telling Ben that he would not be able to convince Bellicus and Serena to help him out today. Also, Paradox puts himself in danger by going into the presence of Bellicus and Serena who almost voted to destroy him, and changes, Ben and himself back into his human form before they could come to a vote.

Ben 10: Alien Force

 * X = Ben + 2 (first and only appearance in Alien Force)

Ben 10: Ultimate Alien

 * The Forge of Creation (first re-appearance)

Video Games

 * Ben 10 Alien Force: Vilgax Attacks

Ben 10: 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)
 * Alien X (Keychain)
 * Alien X (Deluxe)

Ben 10: Ultimate Alien

 * Alien Creation Alien X and Ben
 * Alien X Defender and Vulkanus (DVD and 4 inch action figures)
 * Classic Wildmutt, Nanomech, Spidermonkey Defender and Alien X (Combo pack)

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, being replaced by Cannonbolt/Upchuck.

Trivia

 * Alien X has only been used twice, in X = Ben + 2 and The Forge of Creation making him the second least used alien (tied with Benmummy used once in The Return and Ken 10), Benvicktor and Benwolf are tied for first because they have only been used once.
 * Alien X has a white shadow.
 * Ben said that Alien X is his most powerful transformation in X = Ben + 2.
 * Alien X cannot be destroyed if the personalities agree he can't.
 * When someone asked what gender Alien X was, Dwayne said 'Male and female'.
 * Alien X is the only original alien from Alien Force not to be voiced by Dee Bradley Baker.
 * The extent of Alien X's power is unknown, although Paradox stated that if Ultimate Aggregor absorbed a baby Celestialsapien, he would become omnipotent.
 * Before Alien X appeared, the opening showed only a silhouette of him, after he did it showed his full profile.
 * At the beginning of the video game Vilgax Attacks, Ben starts to turn into Alien X. But Paradox tells him not to yet, but at the end of 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 cannot do anything. After the credits in the last level, we see a cutscene where Albedo is still frozen in Alien X.
 * Alien X is a Nano and an NPC in Cartoon Network Universe: Fusion Fall.
 * in Project Exonaut he is a level 10 suit
 * Alien X is one of the five alien forms that shares the same voice actor as Ben. The other four being Upgrade, Benwolf, Buzzshock and AmpFibian (only in Fused).
 * Alien X is one of the nine aliens to appear on a limited edition smoothy cups in Andreas’ Fault.
 * In Andreas’ Fault, a devoted fan of Ben's refers the Alien X smoothie is a disappointment as well as calling it "filler." This is a reference to Alien X's standing in the series.
 * Alien X is the first alien who have five fingers, the second is Shocksquatch.
 * In X = Ben + 2, Ben did an action first then had to argue. But in The Forge of Creation, he didn't do anything to start the arguing except they have the personalities of alien X almost vote on to destroy paradox when they saw him.
 * The block set on the Ultimatrix that prevents the use of Alien X appeared to have been reactivated after Professor Paradox transformed Alien X back to Ben, for when Kevin absorbs the powers of the Ultimatrix later in the episode to save their lives, he did not receive the abilities of Alien X, thus proving the hacking to be effective with the security system installed.
 * In The Forge of Creation, Paradox somehow turns Alien X back into Ben.
 * Paradox has hinted that Ben will one day use Alien X successfully.
 * It is unknown how the DNA for Alien X was collected since Azmuth does not have access to The Forge of Creation. According to Dwayne, Celestialsapiens also live in the Milky Way, but they're born in the Forge.
 * It is unknown how Ben's association with Paradox affects Ben's relationship with Bellicus and Serena, since Ben is the voice of reason in Alien X and Paradox is not supposed to be within 500 light years of any Celestialsapien.
 * Since Azmuth can detect if Ben and Kevin are hacking the Omnitrix/Ultimatrix, it is safe to assume that he knew that they locked Alien X for security purposes all along. Though Azmuth gets angry every time Ben tries to tinker the device by himself likely without consent, he saw the necessity of hiding Alien X from the active list and presumably allowed them to do so.
 * In Ben 10,000 Returns, Paradox mentioned about an alternate timeline, where Albedo turned into Alien X and is trapped in it for nearly a year.
 * Ironically, in many promotional images of him, Alien X is posing dynamically (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.