The Blood Fort sacrifice is a spell that Tom Riddle claims he set up as a way of taking Hogwarts students hostages on 13 June, 1992. The sacrifice is stopped from activating by Riddle after he successfuly stole the Philosopher's Stone and escaped Dumbledore's trap.
The sacrifice is activated by one or more magic circles that must be set up in advance. Once activated, the sacrifice acts as a field that traps the people inside it and disolves them into blood for the caster to absorb their magical energy. The sacrifice's field can be as large as a stadium.