In what episode of TNG was an ancient Vulcan psionic weapon discovered? If I recall correctly, the device had 3 parts: life, death and balance in between. It worked only against aggression (to the best of my recollection).
The episode that featured the device was called Gambit:
The artifacts are part of an ancient Vulcan telepathic weapon, the Stone of Gol. Picard realizes the true nature of it, once a powerful weapon but utterly useless against people who don't bear aggressive thoughts.
PICARD: Listen to me, all of you! Drop your weapons! Do it! Don't make any aggressive movements. The resonator amplifies violent feelings and emotions. And that's why you wanted me to pick up the phaser. That's how you were able to kill Narik and Vekor. But I can see the symbol on that third artefact, and it is the Vulcan symbol for peace standing between the symbols for war and death. It's a warning that the power of the resonator can be overcome by peace.