Adar Ondi was a dedicated member of Emperor Palpatine's New Order. Born before the end of the Clone Wars on the planet Commenor, Adar was raised to believe in the Galactic Republic. When the Clone Wars were over, however, when the anti-Imperial protests began, Adar's parents were amongst the few that supported the founding of the Galactic Empire. When he became of age, they sent him to the Imperial academy. He was screened for naval service, and when he graduated, the young man was placed on a Tector-class Star Destroyer as a lieutenant. From there, he rose in the ranks, eventually achieving the position of captain by the time the Battle of Yavin took place.
Assigned his own Star Destroyer to command, Ondi was mostly assigned to pirate hunts and law enforcement. However, shortly before the Battle of Endor took place, the man was noted for his abilities and was due for straight promotion to admiral. Unfortunately, when the Emperor died onboard the Death Star, the Empire went into disarray. Uncertain of who he could trust, Adar took his ship and crew, and fled to the Outer Rim territories.
In about five years' time, however, he would come across an Imperial warlord by the name of Keldon Tyfus. Intrigued by the man's position of power, having been a Grand Admiral during the height of the Civil War. However, at this point, Tyfus lost all faith in the Imperial Remnant, and had discarded the traditional white uniform of the Grand Admirals of the Galactic Empire, instead donning a brown-and-black version in its place. Captain Ondi joined Tyfus's forces, and was promoted to the rank of admiral. He helped Tyfus and Surface Marshal Jerel Tremok establish the Shadow Imperium in the Outer Rim Territories, and soon after recruit Phelan Niall, a former Imperial general, as the Imperium's Grand General.
The Shadow Imperium grew, and Tyfus took on a personal project, codenamed Inclement, and assigned Admiral Ondi as the leader of the Imperium's fleet, answering only to the Supreme Commander of the Imperium's forces. Now, Admiral Ondi became the commanding officer of the Shadow Imperium's fleet onboard the Star Destroyer Severus.
During the Battle of Testria, the conclusion of the Sith Revolution, Ondi oversaw the Imperium's forces and did all he could to attain victory. It was futile, however, as even if he did manage to defeat all of Darth Trayus's forces in orbit, the revolution would end on the surface. Ondi retained his rank after swearing loyalty to Trayus.