Higher-dimensional beings who appear as swirling clusters of particles and who, after an accident, rebuilt Spock as a fully human man.
The Kerkhovians are higher-dimensional beings who appear as swirling clusters of linked particles. They are governed by rigid laws that set out exactly how they may deal with other species, and they can rewrite a living being's genetic code in ways Starfleet cannot match.
In Charades, a shuttle carrying Spock and Christine Chapel crashed after an encounter with the Kerkhovians, who repaired the injured Spock but made him fully human. They at first refused to undo the change, so the crew of the Enterprise had to find a way to satisfy Kerkhovian law before Spock's Vulcan half could be restored.