Skin of Evil
The only life form on the planet is Armus, a sinister entity that derives pleasure from the suffering of others.
Picard orders Commander Riker, Dr. Crusher, Lieutenant Commander Data and Lieutenant Yar to the planet to rescue Troi. Upon arrival, they discover that a strange entity has surrounded the crash site with a forcefield, preventing the away team from helping her. Suddenly, for no conceivable reason, Armus kills Yar.
Captain Picard beams down with the rest of the away team in an attempt to negotiate for the crash victims' release. However, Armus responds by ridiculing the crew and tormenting them with vicious and childish pranks.
While Picard tries to reason with Armus, Lt. Worf and Wesley Crusher discover that when Armus is provoked, the forcefield around the shuttlecraft weakens. Picard then taunts Armus, while Lt. Worf and Wesley beam the crew back to the U.S.S. Enterprise.
Picard orders the starship to destroy the shuttlecraft with the ship's phasers and declares the planet off limits to future Federation travelers, thus leaving Armus quarantined on a barren, lonely planet forever.
Returning to the U.S.S. Enterprise, the crew is touched by the living will of Lieutenant Yar. As her holographic image appears before them, she gives a moving farewell to each of her fellow crew members.