Klendth was an inhabited planet where the Loonkerian outpost was located.
Sometime prior to 2286, a piece of bioengineering technology, known as the universal atmospheric element compensator, was made on the Loonkerian outpost on Klendth.
Spock was asked about this technology in his memory test on Vulcan following his "rebirth" on Genesis Planet several months earlier. (Star Trek IV: The Voyage Home)
This planet was only mentioned in dialogue.