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.
X1 and X2 were originally Clones bread in the cloning facilities on Kamino to serve as soldiers in the Grand Army of the Republic. The two of them were both Force-sensitive clones created by the Kaminoans from the DNA of the Jedi Knight, Falon Grey. Grey, at the time in 22 BBY, was the apprentice of Jedi Master Rahm Kota and was badly injured during his travels and was brought to Kamino by his master as a means of helping him heal from his injuries. Grey was healed, but part of his DNA was taken by the Kaminoans without his consent during his time in healing stasis and it was used to create X1 and X2. X1 was considered the "older" brother since he was born an hour before X2 and the two were both trained alongside the other Clone Troopers from Jango Fett's DNA for military service for the Republic.