Yarnek was an Excalbian who, in 2269, diverted the USS Enterprise to Excalbia in order to study Human concepts of good and evil. He arranged for a conflict with Captain Kirk, Commander Spock, and recreations of Abraham Lincoln and Surak on the side of "good," and Colonel Green, Dr. Zora of Tiburon, Genghis Khan and Kahless on the side of "evil."
Yarnek was somehow capable of causing the matter-antimatter shielding to deteriorate remotely. Spock conjectured that his species had the ability to rearrange matter. (TOS: "The Savage Curtain")
Yarnek was played by Janos Prohaska; his voice was provided by Bart LaRue.
Yarnek's name was not spoken on screen, but rather comes from the episode's script.