A Blood Moon is a rare event in which monster spawn rate is doubled, zombies can open doors and several vendors sell new items and have different dialogue. A good example of this is the Gun Dealer selling Silver Bullets and says "Man, I love nights like these, there's never a shortage of things to kill!". Blood Moons also have a 1/9 chance of occurring and can be good sources of income.

These are the effects of a blood moon
Effective tactics against a blood moon are:
1. Barricade your doors so that monsters cant get in.
2. Use wall penetrating items like Vilethorn.
3.Lava Traps
4.Boulder Traps
5.Dart Traps
6.Sharpshooter rooms
7. Bomb holes
8.Grenades
9.Pits

I hope this helped you survive a blood moon,
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If anything needs changed please let me know. :)