Fal-tor-pan was a Vulcan ritual designed to reunite a Vulcan's katra with his or her body.
This ritual was performed on Captain Spock in 2286 after he was recovered from the Genesis Planet by Admiral Kirk.
The ritual was considered very dangerous, both to the recovering deceased person, and to the keeper of the katra. For this reason, it was almost never performed, and at the time of Spock's fal-tor-pan, it was unclear whether the ritual had ever been performed at all. (Star Trek III: The Search for Spock)
A variant of this ritual was performed when the katra of Surak was inside Captain Jonathan Archer. (ENT: "Kir'Shara")