The Yuuzhan Vong is a race of aliens that resided outside of the known galaxy. About twenty-five years after the Battle of Yavin, they began their attack. The New Republic suffered great loss during the invasion. Billions of lives and planets were destroyed. Longtime galactic hero, Chewbacca was one of the first to perish during their attacks.
Despite attacking at a time of the "New Jedi Order," when Luke Skywalker and other Jedis were rebuilding the Jedi forces, the Yuuzhan Vong showed no mercy. The advantage the Yuuzhan Vong had was they seemed to exist outside of the Force.
The Yuuzhan Vong held a deep hatred for technology. To them, technology was blasphemy against the gods. All their weapons and ships actually consisted of living creatures.
Later during the war against the Yuuzhan Vong, the New Republic even joined forces with the remaining factions of the Empire in an attempt to defeat the invaders. The Yuuzhan Vong were defeated and remnants of their army were sent to the sentient planet Zonama Sekot.