what changes when the moon turns red
A blood moon opens at 7:30 PM, closes at dawn, and only reaches the surface. Every body of water in the world goes red, rain included, and a red filter sits over the screen while you are above ground. Beds stop skipping time for the night, though an Enchanted Sundial or Enchanted Moondial still works.
why it is so much worse than a normal night
Three separate things change at once. Surface spawn rates climb, the limit on how many enemies can exist near you climbs with them, and towns stop suppressing spawns, so the safest patch of your map becomes the busiest. Zombies also learn to open doors. On top of that the roster gains members: Clowns that blow holes in your walls, The Groom and The Bride, Dripplers, and in hardmode the Blood Eel.
None of it replaces your usual spawn tools, it multiplies with them. A Water Candle and a Battle Potion during a blood moon produce far more than either does on an ordinary night.
if you are not ready for it
Going indoors accomplishes nothing. The event deliberately spawns enemies next to NPCs, and several of the arrivals open or destroy doors. The real answer is depth: the whole thing stops at the surface layer, so anything below the cavern line is a normal quiet night. Dig down, wait, come back at dawn.
If you would rather hold the base, a block placed in a doorway stops zombies using it. That is worth doing before you ever see the warning, since the message gives you very little time.
fishing during one
Fishing power goes up by ten percent, but that is not why you fish. The event has its own catch table, and it is where the Dreadnautilus, Hemogoblin Shark, Blood Squid and Wandering Eye Fish come from. Those are hardmode enemies that appear straight out of your bobber, and they hit hard enough that a relaxed fishing trip becomes a fight you did not plan for. The Sanguine Staff, Blood Rain Bow and Chum Caster sit behind them, so go in with a real weapon equipped rather than a pole and a swimsuit.
when it flips from problem to opportunity
Once you have full health and a weapon that clears a screen, the same swarm becomes the best coin and drop farm available at that stage. Enemies walk to you, which makes damage over time unusually strong: fire and poison effects early, cursed inferno or acid venom later, all ticking on a crowd that has already committed to closing the distance. A flat arena with two or three platform layers turns the night into a loot run instead of a siege.
It is also the Deathweed harvest. That herb only blooms on a blood moon or a full moon, so its seeds can only be collected then.
small print
- It cannot occur on a new moon, and it will not occur on a night the Eye of Cthulhu is due to spawn. Otherwise the roll is one in nine at each dusk, checked fresh every night, so four in a row is entirely possible.
- Summoning the Pumpkin Moon or Frost Moon over the top of one cancels it.
- Goldfish, bunnies and penguins turn corrupt or vicious for the night. Any released from statues stay that way permanently.
- If it coincides with a full moon, werewolves replace zombies in the spawn pool and nothing else shifts.
- The Zoologist changes to her werefox form, and most town NPCs have their own lines for the night.
- A Bloody Tear starts one on demand and skips every condition except that it has to be night already.
where the name comes from
A real blood moon is a total lunar eclipse. The moon is not lit directly at that point, it is lit by sunlight bent through the entire thickness of the earth's atmosphere, and that long path is what strips out the blue and leaves the red. The same physics is why sunsets are orange. The game kept the color and threw away the astronomy, which is why an in-game one can happen on almost any moon phase.