The Vulcan death grip was an imaginary Vulcan defense fabricated by Spock. When Spock threatens to use the Vulcan death grip when attempting to learn the location of a stolen Protomatter warhead, Scotty feigns shock at the idea that Spock would go that far and later teasingly wondering what they will do if people ever learn that the Vulcan death grip does not exist (Foul Deeds Will Rise).

It did not become a reality until Soleta used it on a Romulan guard to prove a point to Praetor Hiren. (NF novel: After the Fall)

In an alternate timeline, David Marcus asked Saavik to use the death grip on him, to ensure that he didn't reveal any information about the Genesis Device to Kruge. (Star Trek: Myriad Universes novel: The Chimes at Midnight)