The sudden killing of Lieutenant Natasha Yar, security chief of the Enterprise-D, by the malevolent entity Armus on Vagra II during an attempt to rescue the crew of a crashed shuttle.
In The Next Generation episode Skin of Evil, a shuttle carrying Deanna Troi and pilot Ben Prieto crashes on the barren world Vagra II. The away team sent to reach them finds the wreck guarded by Armus, a pool of living black liquid made from the evil that an ancient race cast off and abandoned.
When Yar simply tries to walk past it, Armus strikes her down with a burst of energy for no reason beyond its own amusement. Doctor Crusher cannot revive her aboard the ship.
Picard eventually distracts Armus by forcing it to face its own misery, weakening its hold on the shuttle long enough to beam the survivors out. The crew then gather on the holodeck, where a recording Yar made in case of her death speaks to each of them in turn. Worf succeeds her as security chief.