The Predecessors were a technically advanced race from beyond Our galaxy. They operated from the planet Predecon which notoriously moved beyond known galaxies multiple times. They were first discovered by the Chiss Ascendancy, which had spotted their ships in one of the final battles of the Yuuzhan Vong War. They followed with an Expedition into the Vong Galaxy, after which the Second Predecessor-Yuuzhan Vong War erupted and ended with the destruction of all the Yuuzhan Vong galaxy.
The Predecessors departed for our galaxy before their galaxy was destroyed, and were left to live without a planet for many years. They would join the Chiss Ascendancy in their war with the Galactic Alliance and eventually earn themselves a place in the galaxy on the planet Isolum, which they adopted as their new homeworld. They completely reformed the planet into a place that looked like their former homeworld, Predecon. The Predecessors would later be one of the remaining independent species within the galaxy and joined the Chiss Ascendancy against the Federation of Free Planets.
The Predecessor races were created as part of the Yuuzhan Vong Galaxy, expanding from Predecon to closely located planets like Dag-Ur and Surillia. Many planets fell to the Predecessors and became their colony world, sharing in the Predecessor wealth and resources, most colonies became richer than they ever been.
War beyond the Rims
Yuuzhan Vong galaxy
First Predecessor-Yuuzhan Vong War
Another dominant race aside from the Predecessors were created known as the Yuuzhan Vong, which would expand heavily by use of force and slavery. The Vong would eventually challenge the Predecessors with a greater force, and cause them to retreat along with their planet out of the galaxy, leaving their colonies at harm by the Vong.
The Predecessor race went into self-exile, and would follow their leader, Crank Salueís, rather reluctant. Crank would be removed from the position of army commander and limited in concern of the Predecessor Navy. Crank would eventually lead his species back to the Vong galaxy and silently continued to reclaim former Predecessor worlds, many of which would join with capable ease.
Second Predecessor-Yuuzhan Vong War
The Predecessor Empire would eventually enter combat again with the Yuuzhan Vong, decades later with aid of the Chiss Ascendancy. The Predecessors would however be superior in their technology and eventually caused the Vong galaxy's core to implode, destroying the entire galaxy with it, with the Chiss and the Predecessors only barely escaping to the known galaxy.
Chiss Ascendancy-Galactic Alliance War
The Predecessor Empire would join the Chiss Ascendancy in their war against the Galactic Alliance allying the Imperial Remnant during the Battle of Mon Calamari and eventually attacking joining the other Imperial Alliance members during the final battles of the war.
Following the war, the Predecessors and their empire were awarded a spot on our galaxy to peacefully live on, given that their original world in the Yuuzhan Vong galaxy was destroyed following the Second Predecessor-Yuuzhan Vong War. They relocated to the Isolum system on the edge of known space, subtly expanding into Wild Space. As regard of the Bothan Ar'krai, the Bothans searched for territory where the Predecessors remained, however never finding them due to the distance between the Bothans and the Predecessors.
Infinity WarWhen the Infinity War fired, the Predecessors had formally given up the Predecessor Empire, although the society still acted like the Empire once did. Crank Salueís, the former leader of the Empire, had retired, and a member of House Aruís came into power.
The Predecessors had initially allied themselves with Dark Reach, although the Predecessor leadership had no intention to actively participate in the war. When Dark Reach distrust of the Predecessors grew, their homeplanet, Isolum, was blockaded by a Dark Reach fleet. The Predecessors didn't react well to this, and it escalated into a battle.
When the Predecessors had liberated themselves, they had taken Darth Cyrus captive and prepared for full-scale war against Dark Reach. When the Federation of Free Planets realised that Cyrus was in captivity with the Predecessors, they immediately requested that Cyrus be transferred to the Federation. The Predecessors firmly declined this request, which is when relations between the two got cold.
By 148 ABY, the Predecessors had gained three allies, the Aradan Sectorate, the Chiss Ascendancy, and the Bound Regions, which was essentially a puppet state. The Predecessors had also signed a pact of non-aggression with the Empire of the Hand, sharing the same enemies. Cron Aruís had also suggested that the Predecessor Empire be reformed, although it had yet to happen.