Spoiler warning: Plot or ending details follow.

" Link... Link... Be on your guard. Ganon's power rises to its peak under the hour of a Blood Moon. By its glow, the aimless spirits of monsters that were slain in the name of the light return to flesh. Link...please be careful."
Princess Zelda

A Blood Moon is an event from The Legend of Zelda: Breath of the Wild. Hino is the first character Link meets who explains the phenomenon; from 10:00 p.m. to midnight, based on in-game time, this event has a chance of occurring. The moon and the sky around turn red and the dark aura that is normally seen around Calamity Ganon will be seen everywhere. Its occurrence will cause all defeated enemies to revive from the dead.

The Blood Moon can occur once Link has completed the main quest "The Isolated Plateau" and left the Great Plateau using the Paraglider. During Link's first Blood Moon, Princess Zelda will telepathically warn Link of it and explains that Calamity Ganon's power increases during this time and tells him to be careful. She explains that its glow causes the aimless spirits of monsters that were slain in the name of the light to return to flesh. She will also warn him, in shorter form, each subsequent Blood Moon. This is a point in time in which Calamity Ganon's power will rise to its peak, hence the resurrection of foes that Link has already felled prior to this event. If Link attempts to escape the Blood Moon by passing time by a campfire or cooking pot or by sleeping in a bed, the cutscene will just play before Link finishes or wakes up, respectively. This can frighten players with their guard down, as the sudden roar and shock of red on the screen is very startling. Also, if Link takes refuge at a campfire situated in an enemy camp, the resurrection of the enemies may catch him off guard.

In addition to monsters, any slain Stalhorse will also be resurrected along with their riders. The Blood Moon will also resurrect any slain Yiga Footsoldiers and Yiga Blademasters (save for the one encountered during "The Stolen Heirloom") which is presumably a result of them swearing allegiance to Ganon. Oddly, Master Kohga is not revived after he is slain, though this may be a form of punishment for his laziness and failure to prevent Link from reclaiming the Thunder Helm, and/or Ganon's way of using the Yiga Clan's desire to avenge him to his advantage to motivate them into being more aggressive and dedicated to their mission in defeating Link. Ganon also cannot resurrect his four phantoms the Scourges of the Divine Beasts presumably because it requires to much of his power to resurrect them as they are essentially elemental dopplegängers of Calamity Ganon. Alternatively after they are defeated, Monk Maz Koshia seals their souls within the Illusory Realm preventing Ganon from resurrecting their corporal forms while the Illusory Realm allows them to take on the illusion of a corporal form as part of Link's trials before the Final Trial.

Additionally, the Blood Moon allows Ganon to reconstruct destroyed Guardians (corrupted ones) as well, despite being artificial constructs, though his ability to reconstruct them may be due to the fact that they have been corrupted by him thus can be considered extensions of Ganon himself. Aside from Guardians, a Blood Moon also restores Malice pools connected Glowing Eyeballs, presumably due to Ganon's power reaching its peak. As a result of this phenomenon, Link can never truly rid Hyrule of monsters, Guardians, or Yiga Clansmen. Because Ganon's power reaches it peak, Zelda's seal can do nothing to prevent these resurrections as she is barely containing Ganon by this point.

It is unknown whether those revived by the Blood Moon remain so when Ganon is defeated, at which point the Blood Moon presumably never occurs again due to being an extension of his power.


Link encountering the Blood Moon

However, there are positives to a Blood Moon, such as being able to collect items, parts, or weaponry from enemies that Link has felled without worry of said resources being limited.

Cooking food on a Blood Moon night between 11:30 p.m. and 12:15 a.m. produces better results. Link can ask Hino what Moon there will be that night, and during a Blood Moon night he will state "I'm getting that feeling again... Something is going to happen tonight. I can taste it.". A section of the book "Chef Aurie Taamu, Vol. 2" also describes the positive effects of this phenomenon on cooking.

If Link is inside a Shrine or Hyrule Castle when the clock reaches midnight on a Blood Moon night, the Blood Moon will end without resurrecting any enemies and the cutscene is not triggered. However this generally causes the Blood Moon event to keep on occurring each night until the cutscene is triggered and the enemies respawned.

At Washa's Bluff in the Tabantha Frontier region, Kass plays one of his songs and assigns Link with the Shrine quest "Under a Red Moon", where Link has to stand wearing no armor on a pedestal during a Blood Moon in order to reveal a Shrine.

Shrine Guardian Scouts reset

Despite not being corrupted, Guardian Scouts encountered in most Shrines will respawn after a Blood Moon. Additionally, any completed Minor, Modest, or Major "Test of Strength" combat trial will reset after a Blood Moon event, allowing Link to take on the trial again to combat its Guardian Scout again for its "Guardian" equipment or Ancient Materials. Presumably the Shrines where designed by the Sheikah to reset at that time so the hero could return to them for training or to obtain Guardian Scout spoils which could prove useful to Link in acquiring these weapons to combat the slain enemies which have resurrected by Ganon. The only Guardian Scout that does not respawn is the one used in Ta'loh Naeg Shrine as part of the Ta'loh Naeg's Teachings combat tutorial trial.

Guardian Scout units that patrol certain shrines as enemies are restored as well. Given the Sheikah Monks are implied to have taken part in the ancient war with Calamity Ganon 10,000 years prior, presumably they were well aware of the Blood Moon phenomenon and designed the shrines to reset and restore any defeated Guardian Scouts in a similar manner though do not inform Link of it presumably assuming he would discover it when making return trips in search for treasure or items he was unable to collect upon his initial run. Presumably this itself is a surprise lesson to teach Link the importance of being cautious when returning to areas he explored previously after a Blood Moon has occurred in addition to materials and equipment gained from defeating Guardian Scouts.

Daytime Blood Moon glitch

Rarely a glitch occurs that causes the Blood Moon to trigger during the day and the Blood Moon event to occur at noon (12:00 PM) instead of midnight (12:00 AM). The exact cause of this glitch is unknown, though it is likely due to a glitch in the in-game time clock that causes it to trigger during the day instead of at night when it is supposed to. A daytime Blood Moon has the same effects as a normal Blood Moon. However it can catch players off guard as it occurs suddenly at an unexpected time.

Panic Blood Moon

When the console is using 90% or more of its system memory, a Panic Blood Moon can be triggered as part of a garbage collection routine. This allows the console to clean up its system memory and restore performance to an optimal state. This can occur at any time of day and is otherwise no different than a naturally occurring blood moon.

Spoiler warning: Spoilers end here.

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