The ancient and perilous Vulcan ritual of re-fusion, by which a katra held in another person's mind is restored to its own living body, performed for Spock on Mount Seleya.
Fal-tor-pan, the re-fusion, is a Vulcan ritual that returns a katra to the body it came from. By the 23rd century it belonged to legend; it had not been attempted for ages, and it endangers both the body and the person carrying the katra.
It is performed at the end of Star Trek III: The Search for Spock in 2285. James T. Kirk and his officers bring Spock's regenerated body from the Genesis planet to Vulcan, along with Leonard McCoy, who holds his friend's katra. Sarek asks for the rite, the high priestess T'Lar warns of the danger, and McCoy consents.
The ritual lasts through the night on Mount Seleya and succeeds, although Spock's memories return only slowly. His re-education is still under way at the start of Star Trek IV: The Voyage Home.