Eye of Death

It's believed that the moon is the eye of Decay itself, torn out of the avatar's skull by the First Storm in the ecstatic throes of its birth. Despite being ripped out of its socket, Decay can still use that eye to observe the world. Whenever the moon changes phases, that means Decay is blinking, and during a new moon it is blind to the world. Many tricksters of myth and legend take new moons as opportunities to commit devious acts without Decay's interference.

When death isn't watching, it is unable to collect the dead and thus sends extensions of itself - skeletal shadows known as Keresi or Gatekeepers - down to the world to collect souls in its stead.

Werebeasts Were First Created in the Wake of a Full Moon

It's believed that the first werebeasts used the cover of a new moon to attack one of Decay's keresi with an Orbital Stone. The beasts, not yet requiring the moon's light to become such, devoured the keres with the goal of obtaining its power. Instead, they only served to anger Decay and it cursed them to only become beasts under its watchful gaze during a full moon.

Orbital Stones - Stronger under the Full Moon or just Superstition?

Since orbital stones are considred to be Decay's fractured bone shards and the shattered bones of keres, many mages believe that the light of a full moon empowers the stones even more than they already are. Studies have been conducted to test this theory, but no noticeable difference has ever been observed in an orbital stone's power level. Still though, the superstition persists.