A Blood Moon is an event that occurs during which enemy spawn rates greatly increase. A message 'A Blood Moon is Rising' will pop up on the screen. A Blood Moon has a 1/9 chance to take place when a player has 120 health.
During a Blood Moon, enemy spawn rates double. The power of NPCs stopping enemy spawning in range is disabled. Zombies are able to open doors and the music will turn to that which plays when in the Underworld. Goldfish, Bunnies and Penguins respectively will become corrupt versions of their own selves, and the Groom has a small chance to spawn.
- Doctor Bones also has an increased chance of spawning during a blood moon.