The katra is the essence of the Vulcan mind and can be transferred to another person moments before death.
The existence of the katra was controversial, to say the least; many Vulcans thought it was nothing more than a myth, but a small group of Vulcans named the Syrrannites believed in it. In 2154 an event took place which brought the existence of the katra to the attention of the Vulcan people and its leaders, the Vulcan High Command.
Legend has it that ancient Vulcans used Katric Arks to store katras. Some of those arks were discovered at the P'Jem monastery centuries earlier. Although they were analyzed by Vulcan scientists (one of them even tried to meld with an ark), they were not found to contain a katra.
Surak's katra within the mind of Jonathan Archer in 2154
A highly notable surviving katra was the one of Surak, Vulcan's founder and father of Vulcan philosophy. Although Surak died in the 4th century CE, his katra survived to the year 2154 when it was briefly held by a Human, Captain Jonathan Archer before its transferral to a Vulcan Priest. The reappearence of Surak's katra was instrumental in the rise of T'Pau's influence and the following reorganization of Vulcan government and society.
When a katra is transferred to someone, he or she will benefit from the experiences of the person the katra came from. Because the katra can resist its transfer, the procedure is not without risk and must be performed according to a specific ritual. Non-Vulcans might experience side effects like a form of multiple personality. Humans are especially vulnerable because of the severe shock a transfer can give to their nervous system. Restoring a katra to a Vulcan is preferably done by a Vulcan priest with enough experience with katras. (ENT: "The Forge", "Awakening", "Kir'Shara")
Vulcans can initiate a transfer of their katra by placing one of their hands on the head of the recipient and to position their fingers on specific points. Via a mind meld, a Vulcan is able to tell if someone has received a katra.
The katra can be restored when a person has died and his Katra was transfered to someone, if the family of that person wishes to do so. This ritual is called fal-tor-pan, which literally means "the refusion". This ritual is performed very rarely. The last time was in 2285 when Spock's regenerated body was found on Genesis and returned to Mount Seleya, where his katra was transfered from Doctor McCoy into his own body.
The transfer of someone's katra is not regularly practiced and is only done in special circumstances. When a Human receives a katra, the effects of the merge can by counteracted by lexorin, but this is only for a short period of time. The katra must eventually be transferred to someone else. Around 2369 the existence of the katra was common knowledge within most medical establishments. (Star Trek III: The Search for Spock; DS9: "The Passenger")
Notable katra transfers
- In 2154 the katra of Surak was transfered from Syrran into Captain Jonathan Archer during the aftermath of the bombing of Earth's embassy on Vulcan. Eventually the katra was transfered to a Vulcan priest.
- It is unclear what became of Surak's katra after this.
- In 2285 Spock transfered his katra to Doctor McCoy moments before he entered the radiation room to give the USS Enterprise her warp capability back, so they could flee the Mutara Nebula before the Genesis Device detonated. Spock died of radiation poisoning and his body was "buried at sea" using a photon torpedo as a coffin. The torpedo landed on the emerging Genesis planet, though, where his reanimated body was later found and returned to Vulcan. There, on Mount Seleya the fal-tor-pan ritual was later performed by T'Lar at the request of Sarek, his father, to reunite his katra, carried by Doctor McCoy, with his body. (Star Trek II: The Wrath of Khan; Star Trek III: The Search for Spock)
- It is possible that katra transfers are not complete transfers but rather a copying of information, much like copying a file rather than moving it. Spock was fully functional with all his memories after the transference of his katra to Doctor McCoy which would not be possible if everything he was mentally was completely removed. However, it may be that the reverse is the case; that Spock transfered his katra into McCoy while keeping a 'copy' in his own body to allow him to complete the task he had set himself and save the Enterprise.
Acrophya
In the Star Trek: Vanguard series of novels, the Vulcan science officer, T'Prynn, is a val'reth; she holds the katra of another- her would-be mate, Sten, who she challenged for her freedom in the kal-if-fee- against her will; Sten transferred his katra into her mind as he died, and refuses to leave T'Prynn's mind until she submits to his will. It is, however, unclear whether the term val'reth simply refers to T'Prynn having been given the katra without her consent- in which case, McCoy and Archer could also be referred to as val'reths- or whether she is described as because Sten's katra will not allow itself to be removed from T'Prynn's mind