X1 is a Force-sensitive clone trooper created by the Galactic Republic towards the end of the Clone Wars. A clone created with the DNA of a Jedi Master, X1 was assigned to help the Republic in various missions against the CIS. His younger clone brother was X2, created for the same purpose. However, after the battle on Cato Neimoidia and the enacting of Order 66, X2 decided to leave his dark past behind and fight the Galactic Empire, while X1 chose to stay with the Empire.
X1 and his brother met later on Dantooine. X2 helped Falon Grey, the brothers' genetic father, fight Imperial stormtroopers, but X1 arrived, killed Grey, and left X2 to die. X1 helped the Empire in many ways, including at the Battle of Hoth. The brothers were reunited on a Star Destroyer, and after a short battle, the two escaped the exploding ship.
After the Battle of Endor and the death of the Emperor, X1 became a Sith Lord and operated from Darth Vader's fortress on Vjun. X2, now a Jedi Knight, and the forces of the young New Republic infiltrated the fortress only to find that X1 had escaped to Mustafar. However, using a Sith holocron, X2 tracked his evil brother to Mustafar and attacked the planet in a short battle. X1 attacked his brother above the planet and then in his laboratory of insane clones on the fiery surface. X1 confronted his brother one last time, and the Jedi defeated him, sending him burning into the lava.