Cestus III (mirror)

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

Unbeknownst the $𝜇$ colonists, the planet lay within $𝜇$ 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 $𝜇$s. The Metrons forced Captain $𝜇$ 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 (Through a Glass, Darkly).