You can tell a Blood Moon is out when the sky turns red. There is something about it that causes monsters to swarm.
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, Penguins, 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, sky and water will 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%).
 Blood Moon effects
- 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.
- This also applies regardless of height. This can make Meteor Head-based soul or key farms even more effective, although if you're building in Underground Hallow, you'll have to relocate yourself for the night; otherwise, Chaos Elementals will kill your NPCs.
- 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. An easy fix is to place a block beside the door.
- Enemies can spawn even near NPCs and Home(s), whereas those usually suppress enemy spawns.
- All Bunnies, Goldfish, and Penguins become Corrupt Bunnies, Corrupt Goldfish, and Corrupt Penguins, respectively.
Additionally, in hardmode:
 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, Corrupt Seeds and Crimson Seeds. If you have the console version, she will sell you Sparkly Wings
- 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
- The Cyborg sells advanced ammunition, namely Rocket II & Rocket III (Note: Cyborg has a small chance to sell these ammunitions without a Blood Moon being present. The Blood Moon merely guarantees it.)
- Deathweed will bloom. Blood moons are the only time you can harvest Deathweed Seeds from Deathweed.
- All female NPCs become annoyed. Only the Party girl is not affected by Blood Moon.
- All water will turn a blood red color, regardless of the current biome.
- The Blood Moon's effects on monster spawn rates stack with that of the Water Candle and Battle Potion.
- Whether or not a Blood Moon will occur is determined at dusk each day.
- Every nightfall has an equal chance of being a Blood Moon, and does not change if the previous night was a Blood Moon. It is possible to have two or more Blood Moons in a row and, conversely, a Blood Moon will not necessarily occur even over the course of many in-game weeks.
- In a multiplayer game, using the time adjustment server commands (dawn, noon, dusk, midnight) do not trigger the Blood Moon check, and so using "dusk" repeatedly does not force a Blood Moon to occur.
- Blood Moons is most likely a reference to The Bible prophecy in Revelation 6:12. At the end of the world, 'the sun will become black as sackcloth, and the full moon will become like blood'. In Terraria, however, the world will not end shortly afterwards, but death is still fairly probable, especially to newer players.
- See also Defending Against Events.
- 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.
- An effective way of barricading yourself in is to use Actuators. Instead of placing Doors, you can simply make a 3 block wall, wire them up and use them as a Door instead.
- 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 and actuators 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 (Version 1.2 removed coin drops from both).
- 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.
- During a blood moon, every enemy has a 0.1% chance of dropping the KO Cannon.
- One way to keep your world always prepared for a Blood Moon is to put one or 3 Inactive Stone Blocks next to your doors and wire them to a lever or switch, so when a Blood Moon occurs, you will already have your doors barricaded.
- If you talk to NPCs most of them will have their own unique lines commenting on the event, and female NPCs would be annoyed and rude.
- The Blood Moon is based on one of the "Seven Signs of the Apocalypse" from the Bible, in which it states that ,"...on the 6th sign the moon will arise, showing the color of blood, bringing with it the demons that prowled the Earth, but were sealed away. Then they will feast on those who doubted...".
- The Blood Moon is not entirely fictional. In real life it is an astronomical event in which a full moon passes directly into Earth's shadow - a lunar eclipse. As sunlight passes into the shadow through the atmosphere, the Moon is given a red/orange color due to the amount of air the light passes through. It is the same phenomenon that gives sunsets and sunrises such colors. When a real Blood Moon occurred on the 14th April in 2014 a warning was jokingly announced on the official Terraria forums.
- 1.2: The water during this event now turns blood red, and the KO Cannon now has a chance of dropping from enemies during this event.
- 1.1: The chance of a Blood Moon was decreased to 1/9 from its previous chance of 1/7.
- Pre-Release: Introduced.
| Game mechanics|
|Attack Speed • Breath meter • Buffs • By hand • Consumable • Crafting station • Critical hit • Damage • Data IDs • Day/Night • Death • Debuffs • Defense • Difficulty • Environment • Event • Fall Damage • Game controls • Ghost • Health • Inventory • Knockback • Lighting mode • Mana • Map Size • Minimap • Minions • Modifier • Moon • Music • NPC drops • NPC Spawning • Placement • Player stats • Rarity • Spawn • Storage • Use time • Value • Velocity|
|Random Events||Rain • Blood Moon • Solar Eclipse|
|Summoned Events||Goblin Army • Frost Legion • Pirate Invasion • Pumpkin Moon • Frost Moon|