Val'reth is a Vulcan term given to those members of their kind that possess the katra of another Vulcan in their mind against their will. One known person who suffered this 'state' was T'Prynn, whose would-be mate Sten forced his katra into her mind when she killed him, the katra refusing to leave until T'Prynn had submitted. She was finally 'cured' of this condition via a complex ancient ritual known as the Dashaya-Ni'Var, performed by the Healer Sobon. (VAN novels: Harbinger, Open Secrets)
Notes[]
Given the context of Spock's description, it is unclear whether the term "val-reth" just means that T'Prynn did not volunteer to host the katra, or refers to the fact that Sten's katra will not leave her mind. If the term refers to how they did not choose to host the katra, then Jonathan Archer and Leonard McCoy could also be described as val-reths as neither of them volunteered to host the katras of Surak or Spock, even if they were ultimately willing to accept the situation and pass on the katras later.