From: Star Trek: The Next Generation
Planet: Vagra II. A unique but destructive life-form resembling a living oil slick but able to alter form into a variety of shapes, including a humanoid-like form and a thin shielding large enough to envelop a Galaxy-class starship's shuttlecraft. Armus was created as a byproduct of the Vagran's ridding themselves of their undesirable emotions, and, as the literal skin of their evil, Armus can create formidable energy fields which affect passing vessels, or nearby people. Armus is most known for brutally killing Lieutenant Tasha Yar on stardate 41601.3.