Bloodmoon

A Bloodmoon is an Overworld event in which the event of the same name is announced in the chat. The sky turns red and everything in open air seems to be fully tinted red. During a bloodmoon, the player cannot rest. Monsters spawn at an impossibly dangerous rate preventing the player from withstanding the conditions of outside, forcing them to stay indoors and wait it out. After the bloodmoon ends, all mobs created by it will die as quickly as they surfaced.

There is a 10% chance each night a bloodmoon will occur.

When a player defeats the Wither, they can sacrifice 20 levels to stop a bloodmoon using World Control.