The deaths of Captain Garrovick and some two hundred crew of the USS Farragut at Tycho IV in 2257, caused by a gaseous creature that fed on human blood.
James T. Kirk was a young lieutenant on the Farragut, his first deep-space posting, when the ship met a cloud-like being composed partly of dikironium. Kirk was at the phaser station and hesitated for a moment before firing. The creature killed nearly half the crew, including the captain he admired, by draining their red blood cells.
Kirk carried the guilt for eleven years. In the Original Series episode Obsession he meets the creature again on Argus X and pursues it at the cost of a delayed medical delivery, with the captain's son, Ensign Garrovick, now on his own crew.
Spock shows that phasers could never have harmed it, clearing both men of blame. Kirk and Garrovick destroy the creature with an antimatter charge on Tycho IV.