"Skin of Evil" is the 23rd episode of the first season of the science fiction television series Star Trek: The Next Generation, and originally aired on April 25, 1988, in broadcast syndication. The story premise was written by Joseph Stefano, and the screenplay was re-written by Hannah Louise Shearer. The episode was directed by Joseph L. Scanlan.

The series follows the adventures of the crew of the Starfleet starship Enterprise-D. In this episode, the Enterprise races to rescue Deanna Troi (Marina Sirtis) from a crashed shuttle, which is being held captive by an alien life form called Armus (Mart McChesney, voiced by Ron Gans). In investigating the crash, Lieutenant Tasha Yar (Denise Crosby) is killed by Armus, and Troi is eventually rescued after Captain Picard (Patrick Stewart) distracts the alien.

Yar's death in the episode was the result of Crosby asking to be released from her contract, although her last filmed episode was actually "Symbiosis" as it was filmed subsequent to "Skin of Evil". The manner of her death was intended to be typical of the threat posed to a security officer, and was what the show's creator Gene Roddenberry had intended. The reception of the episode was poor overall from critics, especially the manner of Yar's death and the Armus creature in general.

The Enterprise receives a distress signal from a shuttlecraft returning Deanna Troi from a conference. They find that the shuttle has crashed on a desolate planet, Vagra II, and while they can find the life signs of Troi and the pilot, they are unable to beam the two to the ship. An away team beams down and discovers an animated pool of a tar-like substance, a malevolent life form that calls itself Armus. When Lt. Yar attempts to approach the shuttle, Armus hurls her back with a psychokinetic blast, killing her instantly. The away team is brought back to the Enterprise but the damage to Yar is too great for Dr. Crusher (Gates McFadden) to repair, and they are unable to resuscitate her. A second away team is sent to the planet. Armus taunts the crew members and maintains his grasp of the shuttle. During this time, Troi has communicated with Armus and learned that it is a physical manifestation of evil from the bodies of an ancient race, abandoned on Vagra II.

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