A demon is created by dropping Congealed Blood into a Demon Altar. Demons have many uses.

Right-clicking any block with a demon bashes the demon's head against the block which makes it angrier. It takes 200 right-clicks to reach the maximum anger level.

Shift-right-clicking a torch with a demon creates a Demon Torch.

Shift-right-clicking a log with a demon creates a Demon Item Mover.

Shift-right-clicking an iron block with a demon creates Mob Spawners.

Demons can be used to smelt items in furnaces. The speed of the smelting depends on the anger level of the demon. If the demon is more than half-angry, bad side effects may happen, including loud noises, fire, and randomly teleporting nearby blocks. The amount of side effects also depends on the demon's anger level. A maximally angry demon can smelt items instantly, but you want to keep it far away from your house. Tile entities cannot be teleported, so you can use chests and Demon Item Movers to move items into and out of an instant smelting furnace.

Smelting an angry demon with another angry demon creates a Demon Quarry.

Demons can be smelted into redstone dust. If a demon is used as a fuel source, the fuel demon must be significantly angrier than the demon being smelted; otherwise it creates a Demon Quarry.