Sacrifice is a Ritual added by Roots. When performed, it will inflict 60 damage to the nearest mob (usually killing it) that isn't a player and drop a random plant.
Each Sacrifice Ritual requires a Flint, an Iron Sword and a Bone to be placed on the Casting Altar, and one Blaze Powder and one Dark Oak Bark to be burned in the Incense Brazier. The Ritual requires Attuned Standing Stones to be placed around it.
Runic Tablet entry