The Last Thought (also known as the Killing Thought) was a telepathic technique that was known to the ancient Vulcan people before the Age of Surak.
It was noted as one of the telepathic killing gifts which allowed an individual to kill with just their minds. It was associated with the communications gift (the latter of which was more commonly referred to as psi-communications), but was notably independent of it. A danger with those who possessed the Killing Thought technique was that, when angered, undisciplined minds tended to accidentally kill people through the technique. (TOS novel: Spock's World)
Though it was described as a rare ability, the Killing Thought came randomly every six or seven generations in Old House Yehenik despite their best efforts to stamp it out. The gift appeared in other houses as well (notably House Velekh, in Mahak’s great-grandmother), and could also be achieved through the breeding of two Vulcans with the gift somewhere in their near bloodline, wherein it would be passed onto their child. (TOS novel: Spock's World)
During the years before the Vulcan Reformation, the Lady Suvin of the High House Velekh attempted to breed the gift through bonding a child of Old House Yehenik to her grandson Mahak. Her goal was to get a great-grandchild with the Last Thought technique which she could use as a weapon against her enemies. (TOS novel: Spock's World)
Later in those years before the Vulcan Reformation during a time referred to by historians as the Age of Expansion (when the Vulcan civilisation had just begun to explore and expand into the rest of their own solar system), there was much research into artificially amplifying the killing gift and other psi talents as Vulcan weapons technology became more and more advanced. The Killing gift was refined until it could strike down thousands at a distance of thousands of miles. This was usually at the expense of the adept’s life, but this limitation did little to deter the gift or those who bred for it, as most with this talent only used it under threat of harm to themselves or their families. (TOS novel: Spock's World)