I've been casually playing Terraria for some time. I really wanted the blood moon to spawn but I have been playing for some time and only one has appeared.

How often do they appear?

  • Make sure you have found at least one heart crystal, so your max HP is at least 120. – Hugo Zink Jun 1 '15 at 18:25

From Terraria Gamepedia:

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.

Following the criteria above, there is a 1/9 chance (or 11.11%) chance of a Blood Moon occurring.

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