Blood Moon

An occasional event in Terraria which causes the moon and the sky to turn red. During a Blood Moon, enemy spawn rates double, Zombies can open doors, and Goldfish/Bunnies turn corrupt, exposing any NPCs in the path of the monsters to harm. Each night has a 1/7 chance of the Blood Moon rising. This chance is increased when a water candle is possessed by a player. When Night comes and it's a Blood Moon, A message will appear saying "The Blood Moon is rising..." Fighting monsters on a Blood Moon is a great source of income if your character has at least mid-level equipment (iron or better is advised). Others who are not so fortunate and wish to survive may wish to block Doors and seal off entranceways. Blood Moons have been known to last up to 3 Days. If a 2x2 trench is dug in front of a door, zombies cannot open it.

A simpler way to block out zombies is to place a wood platform against the top 3rd of the door opposite the outside. You can open the door one way as much as you want, but zombies will not be able to open it the opposite way. NPC's can still use the doors effectively.

There is also a trick where you can put one block in front of the door and open it while the zombies are outside. since zombies follow the Fighter AI, when they realize they can't get to you, they will retreat and despawn. although this doesn't give you any loot, its a quicker way to get rid of zombies.

During a blood moon, NPC's will also display unique dialog. Here's a list of quotes that can only be seen during a blood moon.

"Keep your eye on the prize, buy a lense!" - Merchant

"You can tell a Blood Moon is out when the sky turns red. There's something about it that causes monsters to swarm." - Guide

"I've got something for those zombies, alright!" - Demolitionist

"It is an evil moon tonight. Be safe." - Dryad

"I love nights like tonight. There's never a shortage of things to kill!" - Gun dealer

"[Player name]...We have a problem! It's a blood moon out there!" -Clothier