The Tellun system is a star system located on the Klingon-Federation border. The Tellun system was home to two inhabited planets under Federation control, known as Elas and Troyius, but also claimed by the Klingon Empire.
Unknown at the time to the Federation, Elas was a rich source of dilithium, a fact the Klingons had full knowledge of, and was the basis of their claim on the system. An interstellar incident nearly occurred in the system between the two powers, in 2268, when the Klingons attempted to assert their claim on the region by having the USS Enterprise sabotaged by a Klingon sympathizer aboard, the Elasian guard Kryton. The sabotage was averted, and it was later discovered that the Dohlman's radan necklace, a gift from the Troyians, was, in fact, crude dilithium, saving the Enterprise and thwarting the Klingon's intentions. (TOS:"Elaan of Troyius)