Season 1 Episode 23
Episode: "Skin of Evil"
After having survived a shuttle crash on planet Vagra II, Counselor Deanna Troi finds herself being held ransom by the sole life form that resides on that world; a malevolent entity called Armus, a creature that presents as an oil slick. Armus offers no regard or concern to the away team sent to rescue Troi, indeed after having existed for countless centuries in isolation what he apparently wants is to be entertained, subjecting the away team to his talents for torment and cruelty.
What's this now? - Did Armus just kill off a main character? -- Indeed it is so.
Tasha Yar quickly tires of Armus's bullshit and attempts to barge through the line that Armus maintains between the away team and Troi's crashed shuttle. With no hesitation, Armus retaliates and executes Tasha on the spot.
