A living mass of black, oily matter left behind on Vagra II, formed from the evil an ancient people cast off, which killed Tasha Yar.
Armus was a shapeless creature of black liquid that could rise into a roughly humanoid form. It said it had been created when the inhabitants of Vagra II found a way to shed all their evil, then left that discarded part of themselves behind on the barren world. Alone, it nursed a deep and cruel resentment.
In Skin of Evil, Armus trapped Deanna Troi inside a crashed shuttle and casually killed Lieutenant Tasha Yar when an away team tried to reach her. Picard kept it talking while the crew rescued Troi, then left it stranded on Vagra II with orders that no one ever return.