Here's what Doug Beyer had to say about the Helvault when he previewed the card:
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.
But did you know that Dark Ascension was not the first time that the Helvault was shown? Look what happens if we swing around to the other side!