Aurelion

Aurelion has two subspecies of the same intelligent life forms, very much like our notion of Alien visitors. The shorter of the two are very pacific and the taller of the two run the show (administrative types). An invading force makes a deal with the administrative types to turn the defective citizens of the shorter class into cyborg explorers. The measure passes by a slim margin but the invading force almost immediately welches on the deal.
The Administrative types met in secret counsel and approved the use of the power of the smaller citizens. They are gifted in the use of 'magic' a planetary power that the administrative types cannot access. The invading force have no idea what is happening and pack up to leave and take 40, 000 of their cyborgs with them, their power and systems no longer work. 20, 000 were herded into the 'southern zone' and kept captive behind a barrier they could not penetrate.
Eventually the strongest of the mages created a virus to eliminate them, their comrades had no intention of attempting a rescue, "We've got a galaxy to explore, exploration involves risk, you are just collateral damage." Aurelion rebuilds and changes its covenant to include the smaller subspecies of citizens.