The Vel'kiran Empire was founded as a continuation of the rulership of Til'raine. Each city-state was governed over by a mage of god-like power who constructed and stabilized a region of the vast plane around their Nexus. Although the realm was immense, it was possible to simply walk between the city-states, and over time their influence grew and their borders expanded, and many settlements sprang up in the wilds between the kingdoms.
Ultimately, this was its downfall. While each Nexus bore powerful magic to stabilize the region of the plane they were near, the protection of the lands between the Nexi was not nearly so strong, particularly the further away from a Nexus one went. Vel'kira was unstable from the moment it was created, but continued experiments with Void Magic caused the entire plane to destabilize.
With the breaking of the planes, the influence of the Vel'kiran Empire, was forever destroyed. In truth, it had been on the decline for millennia, as the god-mages disappeared one by one. Some died, some abandoned their city-states and went off into the wilds or back to the Prime Realm, others simply vanished without explanation, possibly due to Void experiments. A few of them even went mad and snapped, and destroyed their own section of the plane, which did not help with its stability at all. In the end, no one was able to govern what was once a mighty empire any longer.