Eternal Blood Moon

Contrary to popular belief, lycans (or werewolves) aren't limited to transforming during a Full Moon. Lycans transform on whatever moon phase they were infected on. If infected with Lycanthropy on a First Quarter moon, the infected would turn during the next First Quarter, and so on.
Infection runs rampant if a the moon phase lasts several nights such as a Waxing Gibbous. Entire packs of lycans would transform in a span of many such night phase. They would terrorize the countryside, towns, and sometimes cities. Devouring livestock, humans, and further spreading Lycanthropy.
The Silver Bullet Era stemmed the tide of these beasts (which still roam to this day). There exists the legend of the ultimate moon phase, the Eternal Blood Moon.

Origin

Only vague texts between 0-112 AE (roughly 1400 years ago) state that an Eternal Blood Moon was responsible for throwing the world into the hands of the lycans for the first time. The authors of these texts are unknown. The language barely translatable. The text themselves were found in the highest moutains of Caledonia, in some extinct language barely spoken anywhere in Mercial.

Eternal Blood Moon

According to the text, the Eternal Blood Moon is a crimson red, Full Moon phase. It coats the moon in blood red, casting the world in a crimson hue. The entire world is plunged in a void of darkness and blood red for one month. While the moon doesn't last an eternity, it certainly feels like it.
During the Eternal Blood Moon, every single lycan will transform regardless of their moon phase. Full Moon, Waxing Crescent, Waning Gibbous, New Moon, and more. All transforming at once and launching a nocturnal war of blood and rage.

Myth or History

It is difficult to determine whether the Eternal Blood Moon has ever taken place. Citadel scholars deny the Blood Moon ever happening, stating that if it were to happen then humanity would have truly died in the First Era. No amount of silver of wolfsbane would be enough to kill so many lycans across the world.
Others believe the Blood Moon did occur once sometime in the First Era. That could only explain how Lycanthropy became so prevalent in the first place and why the disease has managed to stay rampant thousands of years later.