Alien X is a Celestialsapien (celestial referring to a supernatural being and sapien from the Latin wise), native to their Forge of Creation, their birthplace, outside the known Universe, hidden from everyone aside, except Paradox and who possess the Map of Infinity.
Alien X has a black body resembling space with tiny white stars, and a white shadow and white hands. He has the power to be omnipotent, being capable to warping reality at a will; they are all usually in an inactive state of internal debate due to two conflicting personalities: Bellicus, the voice of rage and aggression, and Serena, the voice of love and compassion; while Ben is the voice of reason. All three speak together when Alien X speaks. But to perform any action, including but not limited to, simple move to turn back, 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. Though the two are usually unable to agree on anything, they make an exception in the case of Paradox as they both hate him to the point of allowing him to exist only if he stayed "500 light years" far from them, or any Celestialsapien.
Alien X is the tenth alien that Ben becomes after his Omnitrix was reformatted when he stops a dam that had burst in "X= Ben + 2", becoming a metaphysical medium for both Bellicus and Serena to settle their debates as their voice of reason while physically unable to move as Alien X. When none of his ideas worked, Ben gave them the idea to let him become a different alien to fight the Incursions. Since then, though he acknowledges Alien X as one of his most powerful aliens, Ben cannot afford the cost that comes with assuming the form, the reason being that unless he can get either persona on his side, leaving Alien X's body trapped in place.
After getting the Ultimatrix, Ben had a security protocol that lock on the Alien X option to ensure the form would not be used if the item fell into the wrong hands and that he would not accidentally turn into it, Gwen and Kevin have the keys that undo the protocol. Ben had this undone in order to get Bellicus and Serena to help him to find the Forge of Creation and stop Aggregor, which didn't worked. Paradox ended up getting Ben out of his Alien X form after the boy's attempt to reason with the two personalities fails prior to them realizing Paradox's presence and were about to vaporize him upon a quick mutual agreement.
