The planet where a vampiric cloud creature killed half the crew of the USS Farragut in 2257, and where Kirk finally destroys it eleven years later in the episode Obsession.
Tycho IV is central to the Original Series episode Obsession. In 2257 the USS Farragut met a gaseous creature there that fed on red blood cells. It killed some two hundred of the crew, including Captain Garrovick, and the young Lieutenant Kirk blamed himself ever afterwards for hesitating at the phaser controls.
Eleven years later Kirk meets the same creature on Argus X and pursues it across more than a thousand light years. Spock works out that it is returning to Tycho IV, where it apparently came from, in order to reproduce.
Kirk and Ensign Garrovick, the dead captain's son, beam down with an antimatter bomb and use themselves as bait. The explosion destroys the creature and tears away part of the planet's surface.