When Star Trek: The Next Generation began airing on first-run syndication in 1987, it was a rarity for the franchise to permanently kill off any leading characters. So it came as a huge surprise when Lt. Natasha Yar met her untimely death in Season 1, Episode 23, "Skin of Evil," in which the USS Enterprise-D attempted to rescue Counselor Deanna Troi (Marina Sirtis) from a crashed shuttlecraft held hostage by an evil entity called Armus. In a demonstration of his power, Armus killed Yar while she was investigating the crash site. Yar's death weighed heavily on the crew, who gathered shortly afterward and held a memorial service for the fallen Chief of Security in the ship's holodeck. Lt. Worf stepped up to become the Enterprise's Security Chief, a position he held for the remainder of the series' run.
Source：https://www.cbr.com/star-trek-why-denise-crosby-tasha-yar-left/
