In the mirror universe, Tharn was the leader of the Halkan Council. In 2267, he met with a landing party from the ISS Enterprise led by James T. Kirk to discuss allowing the Terran Empire rights to mine dilithium on the planet Halka. On behalf of this people, Tharn refused to allow the Empire access, due to dilithium's destructive potential.
Once denied, the Enterprise had standing orders to obliterate the Halkans. However, the Captain James T. Kirk of the primary universe, who had replaced that universe's Kirk, contacted Tharn and gave him twelve hours to reconsider. Tharn refused, telling Kirk that twelve thousand years would make no difference to his decision. When threatened, Tharn spoke of his entire race's willingness to die to maintain peace, and foretold the rising of other slave worlds against the tyrannical Empire. (TOS episode: "Mirror, Mirror")
In the trilogy of books written by William Shatner and Judith and Garfield Reeves-Stevens, Spock successfully convinced Kirk to spare the Halkans, using the fiction that a secret alliance existed between the Halkans and the Klingons, so that if the Empire moved against that one planet it would give the Klingons an excuse to declare war. Kirk received honors for avoiding such a devious trap. Tharn's fate in that universe is unknown, but the Halkan homeworld did not fare well once the Klingon-Cardassian Alliance took over.