The Battle of Ygziir was the end of the Cremlevian War, a conflict that was waged in the Yuuzhan Vong Galaxy millennia before the Battle of Yavin. One faction of Yuuzhan Vong was led by warmaster Steng, the other by warmaster Yo'gand.
The planet Ygziir was home to a domain loyal to Steng. Ygziir was attacked by Yo'gand's forces. Yo'gand used powerful dovin basals to pull Ygziir's moon into the planet, destroying both. At some point during the battle, he fought Steng in personal combat and killed him. The Battle of Ygziir was the last conflict of the Cremlevian War. Following the battle, Yo'gand became sole warmaster of the Yuuzhan Vong. Steng's faction gave up resistance (or were wiped out), and the race was unified, with Yo'gand becoming the first Supreme Overlord of the Yuuzhan Vong. The Yuuzhan Vong galaxy had been destroyed over the course of the Cremlevian War. Following its end, and the unification of the Yuuzhan Vong, Yo'gand decided to leave their galaxy for elsewhere. Also, the tactic of crashing a moon onto its planet later became known as the Yo'gand's Core tactic, named for its inventor.