Blood Moon



A Blood Moon is a common, randomly occurring event that takes place over the course of an entire night. It causes several hazards for its duration: Enemy spawns increase, even near your Home and NPCs (where enemies do not normally spawn), Zombies become able to open Doors, and Bunnies and Goldfish transform into dangerous enemies, along with other effects (see below). Blood Moons are signaled by the message "The Blood Moon is rising..." in the lower left corner of the screen, and the moon and sky appear red for their duration.

A Blood Moon will only occur if at least one player in the world has over 120 life, and only on nights when a moon is present (any night except a new moon). Given those criteria, a Blood Moon has a 1 in 9 chance (11.11%) of occurring. Without taking the current moon into consideration, there is an overall chance of approximately 1 in 10 (1/9 * 7/8 = 7/72, or 9.72%).

Enemies
Additionally, in hardmode:
 * The spawn rate of monsters on the surface increases, and The Groom has a chance to spawn. Enemies not only spawn faster, but more are spawned than can normally exist on-screen at once. This works no matter the environment, so Corruption enemies can also swarm even more than they normally would, for example.
 * Zombies become able to open a door after hitting them two times. This can put the player's home and NPCs in peril if they are not guarded appropriately.
 * Enemies can spawn even near NPCs and Home(s), whereas those usually suppress enemy spawns.
 * All Bunnies and Goldfish become Corrupt Bunnies and Corrupt Goldfish.
 * Possessed Armor can open doors, similarly to zombies.
 * Clowns have a chance to spawn.

Other effects

 * The store Inventory of most NPCs change:
 * The Merchant will sell Throwing Knives
 * The Arms Dealer sells Silver Bullets
 * The Clothier sells the Mime Mask
 * The Dryad stops selling Purification Powder, Sunflowers, and Grass Seeds, and starts selling Vile Powder and Corrupt Seeds
 * The Painter sells Evil Presence painting
 * The Steampunker sells Purple Solution (if the Corruption is present) or Red Solution (if the Crimson is present) instead of Green Solution
 * Deathweed will bloom. Blood moons are the only time you can harvest Deathweed Seeds from Deathweed.
 * Water will become blood red.

Tips

 * Newer players may want to barricade themselves indoors during the event. Doors can be barricaded easily by placing blocks of any type directly next to them. Other placeable items like the Table or a Torch will prevent doors from swinging inwards, which is the only way Zombies and Possessed Armor may enter. Placing all doors at least one tile above the ground, or with a 2x2 trench in front of them, will also prevent intruders. The player can also place two doors next to each other to prevent the intruders from entering.
 * Blood Moons can benefit a fairly well-equipped player by providing quick money and items through enemy drops.
 * With a Flail, open a door, throw the flail, and quickly close the door so the flail is stuck outside. Any approaching enemies will be hurt/killed on contact.
 * More advanced players may want to use Mechanisms to construct lock-down systems to barricade entrances, and traps such as Lava "moats", to deal with Blood Moon and Goblin Invasion enemies. Active Stone Blocks can make simple or elaborate systems possible.
 * With a Fish Statue or Bunny Statue and a 1 Second Timer, Corrupt Goldfish or Corrupt Bunnies will continuously spawn. This stops other enemies from spawning, and the player is able to make money by killing those relatively easy enemies. Corrupt Bunnies also provide the Bunny Hood, which sells for 40 silver (DOES NOT WORK ON 1.2!).
 * Using a Bunny Statue or a Fish Statue during a blood moon while in the underground hallow or corruption is a great way to quickly gather large amounts of Souls of Night and Light.
 * Being near surface Hallow or Corruption means large spawns of those types of enemies during Blood Moons. This is useful for farming Mimics, since they drop so many rare items. This method also produces very large quantities of Pixie Dust, which is used in Greater Healing Potions.

Trivia

 * Female NPCs become unusually annoyed during Blood Moons (the Nurse and the Mechanic especially). This is most likely a clever joke based on "blood" and "that time of the month."