Tallera

Tallera was a Romulan mercenary, and the alias for a Vulcan named T'Paal.

In 2369, Tallera infiltrated Arctus Baran's raider. She confided in Jean-Luc Picard who also had infiltrated the mercenary ship, that she was a Vulcan operative sent to find the Stone of Gol. It amplifies telepathic energy and if used by a trained telepath, it becomes a powerful weapon that can kill. An isolationist group on Vulcan was seeking the weapon to overthrow the government of Vulcan.

In reality, Tallera was the isolationist and upon finding the pieces of the Stone of Gol (a psionic resonator), assembled it and killed the mercenaries on the ship. When she tried to kill Picard, he realized that the stone used aggressive energy of the victim to kill. By clearing his mind of violent thoughts, he was able to disarm Tallera and arrest her. 

Tallera