In the mirror universe, Cestus III was the site of an imperial colony of the Terran Empire.

Unbeknownst the Terran colonists, the planet lay within Gorn space and was subsequently attacked and occupied by the reptilian race. Imperial Starfleet launched a retaliatory raid on the planet which was interrupted by the arrival of advanced alien beings who referred to themselves the Metrons. The Metrons forced Captain James T. Kirk and his Gorn counterpart into a duel to the death. When Kirk hesitated to kill the Gorn, the Metrons took it to be a sign of mercy and weakness on the part of Terrans. Kirk and the empire were then banished from Gorn space through the appearance of a vast energy barrier created by the Metrons. (Decipher RPG module: Through a Glass, Darkly)