Avacyn - sealing

One by one, Avacyn and her host of angels captured the demons, imprisoning them in a huge mass of silver that came to be called the Helvault. Said to be a piece of Innistrad's silver moon, the Helvault sits in the cathedral yard in the High City of Thraben overlooking a cliff's edge and the sea beyond. Stories say it and Avacyn first appeared on the same night, and it remains the most holy object on Innistrad after Avacyn herself.

With the proper incantations and rituals, Avacyn and her angelic host could cause wicked creatures to be drawn into the Helvault. She trapped so many demons within the Helvault that only a handful remained. To the relief of all but the demon-worshipping Skirsdag cult, demons and devils were largely out of human life.

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THE PRISON OF SILVER