Beware major plot spoilers for the entire Amnesia series, together with descriptions of fictional torturing techniques.
The most ghoulish of Amnesia's many ghoulish ideas is "vitae". It's a luminous, blue-green fluid that can be used as an energy source, a chemical catalyst and a supernatural healing agent. Each Amnesia game is broadly a story about the production of vitae, with some levels consisting of huge distillation and refining apparata, and major plot developments tied to what you do with it. In the original Amnesia: The Dark Descent, you'll brew potions using vitae in order to keep a decapitated head alive. In 2020's Amnesia: Rebirth, you'll use ornate metal cannisters of the stuff like batteries to power otherworldly machines. Vitae is the blood and breath of Amnesia, the grease upon the narrative axels, the fuel in its boiler, the miracle McGuffin that sustains its nastier flights of fantasy. But what is vitae, exactly? Agony.