Blood moon

The Blood Moon is a game mechanic that ensures the world stays populated with enemies and weapons. Every time a blood moon occurs, enemies that have been defeated and overworld weapons that have been picked up by the player respawn.

Blood moons are also used to reset internal state when some subsystems are running out of memory or become unresponsive. Such blood moons do not follow the regular schedule and are commonly referred to as "panic blood moons".

A common misconception is that blood moons help replenish system memory by resetting enemy kill flags. This is however total nonsense, because enemy kill flags are just GameData flags, and all GameData flags are loaded at bootup and stay in memory forever.

Scheduled Blood Moons

Full article: WorldMgr

Scheduled Blood Moons are based on a timer that starts running as soon as the "step out of Shrine of Resurrection" (Demo103_0) has finished. When the timer reaches 7 in-game days, the WM_BloodyDay flag is set. The next time it is midnight, assuming blood moons are not inhibited, the Blood Moon cutscene will trigger. For more details, read WorldMgr.

Panic Blood Moons

Panic Blood Moons occur when the game is running out of memory or when some tasks are taking too much time.

Contrary to a widespread theory, they are not used as a generic error handler or a fallback for "unhandled events". Panic blood moons can only occur in specific cases.

Panic conditions are checked every frame by GameScene, which also keeps track of the panic reason in an unsigned 32-bit integer so that memory issues can be reported to Nintendo via the telemetry system.

Panic Blood Moon reasons

Bit	Description
0	Resource system: at least one of the following is true: Bit 12 is set in the ResourceMgrTask's flags (indicating a problem with a resource load[check]) An overlay arena's heap free size percentage is less than its configured minimum percentage. The limit is 0% for the OverlayArena system itself, "Tera SZS work" (Tera decompressing buffer), Audio and StarterPackMgr. The default is 0% too. The limit is 5% for the "ForResourceS" (small) and "ForResourceL" (large) heaps, which is where most resources are allocated from.
1	PhysicsMemSys: Havok main heap is running out of memory (less than 5% free)
2	PlacementMgr: Actor spawning heap is running out of memory (less than 5% free)
3	ResourceSystem/OverlayArena: ForResourceS heap is running out of memory
4	ResourceSystem/OverlayArena: ForResourceL heap is running out of memory
5	ResourceSystem/OverlayArena: Audio heap is running out of memory
6	ResourceSystem/TextureHandleMgr: Last TextureHandleMgr::calc execution took more than 60 seconds[check]

A panic moon causes enemies and other respawnable objects to respawn since it calls Demo011_0 just like the regular blood moon code. The demo contains a call to AIDef:Action/EventOffWaitRevivalAction (OffWaitRevival), which appears to be what actually resets the revival flags.