Terraria Guide: The Blood Moon
When night comes and it is a Blood Moon, a message saying The Blood Moon is rising...
The moon and sky will become red, and the water will be red.
Enemy spawn rates double, and the maximum number of enemies able to be spawned at once increases. Zombies gain the ability to open doors and Goldfish and Bunnies will turn corrupt.
Roughly 1 in 70 zombies spawned on a Blood Moon will be a Groom instead, and Doctor Bones becomes easier to find. If Hard Mode is active, Blood Moons enable the spawning of Clowns as well.