During the late 21st to the early 22nd centuries, The Terrans had been using Terraformer machines in order to make planets habitable, and when no longer needed these were often abandoned and left to float through space.
In time, the Terrans cancelled the project, and tried to destroy the machines with a self destruction patch. However, a disgruntled chief engineer discovered that the CPUs were becoming sentient, and managed to intercept the patch before it could be fully deployed, and modified it so that the Terraformers would survive and eventually become fully sentient. Unfortunately, it had an unexpected effect: It made them highly aggressive and vengeful against Humanity. This caused a horrific war that nearly destroyed the Human race. In 2146, the proto-Xenon pursued the six surviving USC capital ships into the man-made jumpgate that connected Brennan's Triumph and the Solar System, which was destroyed shortly after they crossed it. They have since evolved into the Xenon.
Some of the TF/CPUs were not infected by the Xenon Virus, and stayed loyal to humanity, like #deca, who resides in Collins' orbit, in the Solara System.
Terraformers are a form of AGI and are currently banned by the Terrans.