Somme 2.5 millions years ago, The Xu-Ha were one of the most advanced and respected species in the galaxy. They had been one of the chief architects of their era's galactic peace and they were beloved by many.
Transcendence drove the race mad. This madness was a slow cultural tide that took millennia to truly manifest but the signs were clear in retrospect. The Xu-Ha had failed as a Third Age race; without no transcended species to guide them, they were adrift.
For no apparent reason, the Xu-Ha began purging the galaxy of lesser races. Their actions were subtle, involving hidden machines that blasted out psychic pulses which only affected pre-Transcendence races. It took centuries for the increasingly isolated elder races to even notice that their lesser counterparts were dying off in droves, and it took even longer to discover the source. The elder races offered the Xu-Ha a chance to stop their campaign of genocide. The Xu-Ha refused this offer so the elder races acted as they saw fit.
The Xu-Ha were annihilated, their existence wiped from the galaxy.
Source: Babylon 5 universe, Darkness & Light: The Vorlon & Shadow Fact Book, Mongoose Publishing