Alien X is one of the alien forms that the protagonist, Ben Tennyson, can transform into in the animated television series "Ben 10". This form is part of the transformations available in the Omnitrix, a device worn by Ben that allows him to transform into various alien species.
Alien X is a Celestialsapien, a species known for their god-like powers. The character is shown as a humanoid silhouette filled with stars and two starry energy streams coming off its body, representing the character's cosmic nature.
The use of Alien X's powers is constrained by the fact that the Celestialsapien species has three personalities, and at least two of the three must agree before Alien X can take any action. These personalities include Serena, the 'voice of love and compassion', Bellicus, the 'voice of rage and aggression', and Ben, the 'voice of reason'. This limitation creates a considerable challenge for Ben, often leading to him avoiding the use of Alien X unless absolutely necessary due to the potentially catastrophic consequences of inaction.