The psionic lens is a xel'naga artifact that amplifies the psionic power of whoever wields it. In addition, even if the wielder had no psionic potential, the artifact links directly to the wielder, allowing them to use psionic powers from the artifact itself.
The dark templar guardian Lassatar was tasked with retrieving this artifact on the terran colony world of Roxara. He could not locate it, but a terran child named Tiffy guided him to the relic. However, he sensed that Nero, a reaper, had come to kill her father, and slew him. He then took the artifact and went offworld.
He took the relic to an order of acolytes who studied it, realizing its potential. They then took the relic and a group of zerg to an uncolonized planet and began studying how to uplift the zerg so that they wouldn't be a threat to the protoss. They sought to disconnect them from the hive mind, and give them emotional responses. This worked, but backfired, and the zerg reverted to their now amplified primal instincts and destroyed the acolytes.
Lassatar came to the planet on the trail of the relic, and retrieved it, discovering what the acolytes had done. He ran into a human named Jake who had stumbled on the world, and the two tried to flee the zerg. When a mutated ultralisk attempted to kill Jake, Lassatar used the lens, gaining power equal to an archon, and slew the monster. He was stunned by the feedback, forcing Jake to save him.
Lassatar then used the relic to give Jake an image of the entire volcano the base was in, as well as the points that would cause the volcano to erupt if explosives were placed. He also used the lens to make Jake feel great joy for working to accomplish this tasks, locking him on the path of destroying the zerg. Jake used MULEs to drill into the mountain, and destroyed the zerg inside. Lassatar took the relic and said farewell to Jake, leaving the planet.