The Helvault
The Helvault is a huge silver mass that stands at the precipice inside the courtyard of the Cathedral of Avacyn in the city of Thraben. Its surface is rough and unrefined, and thin veins of dark mortar branch across its surface.
Avacyn's campaign
Edgar Markov created vampirism on Innistrad thanks in part to the prodding of a fiendish figure called Shilgengar.
Shilgengar was one of few demons to exist on the surface of Innistrad at the time, and he became a kind of harbinger for his own kind. Edgar inflicted his curse of vampirism on his grandson, Sorin, awakening Sorin's latent planeswalker spark. And later, Edgar's actions indirectly led to a resurgence of demons in the world.
Now a planeswalker, Sorin lived and traveled and fed his vampiric thirst for decades, and then the decades stretched into centuries. When he returned to Innistrad and saw the threat to life on Innistrad that vampires and other supernatural monsters represented, Sorin took action. He created the archangel Avacyn to safeguard humanity, providing both vampires and humans a way to subsist in the world.
The archangel Avacyn and her host of angels took up the sword against those who would harm humanity. They slew vampires with holy fervor, thrashed the werewolf howlpacks that threatened to overwhelm those towns bordering the wilds. They banished malevolent geists and they incinerated unholy ghouls with piercing light. They were never able to cleanse the plane of evil, but Sorin's creation had done her job.
Humanity was able to flourish again and the power balance on the plane tipped in humanity's favor. None could have foreseen what evils this would bring.
Demons and Devilry
The retreat of the supernatural fiends (vampires, zombies, werewolves and geists) of the plane opened a space, an opportunity for Innistrad's infernal forces to manifest. Like Shilgengar long before, the demons and their hosts of impish devils began to make their presence known in far greater numbers.
Avacyn engaged each demon in single combat, defeating them one by one. She found that new demons would appear a short time later and decreed: "What cannot be destroyed must be bound." She then forged a collar of silver to bind demonkind, so they could be hauled away and imprisoned. The Silver Collar became Avacyn's symbol and the holy symbol carried by Avacynian priests and cathars.
The Helvault imprisonment
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 lake 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.
It was only a short time ago that the demon Griselbrand, the most powerful of his kind and one who had never become imprisoned, committed an act of unprecedented boldness and heresy: he landed on the Helvault itself, under a full moon, and challenged Avacyn to combat.
Avacyn and Griselbrand battled for days, out of view of all but Avacyn's host; the Lunarch, Mikaeus; and his most trusted bishops. Finally, in an act of desperate effort, Avacyn used all of her remaining strength to drive Griselbrand into the Helvault. As his final act, Griselbrand threw his spear and impaled the archangel's heart. The binding spell backlashed as she was impaled, and Avacyn was drawn into the Helvault herself.
Innistrad's most powerful demon was now encased in a great silver mass, as was the archangel Avacyn herself, keystone of the Church and protector of humankind. The words of her own spell imprisoned her: "What cannot be destroyed must be bound."
The waning of the light
Avacyn was gone. The Mikaeus reeled. Church officials struggled with what to tell the general populace, and whether to keep Avacyn's imprisonment a secret. As the days and weeks went by, practitioners of the faith felt the effects. Rumors abounded that Avacyn had abandoned the world, leading to a crisis of faith across all four provinces. With humanity's safeguard trapped within her own silver vessel, the people of Innistrad are now left exposed to the creatures of the night.
Destruction
Liliana Vess, who had made a demonic pact with Griselbrand, sought to destroy him, and thus was the cause of the Helvault's destruction. She proposed a threat to Thalia, the Helvault's guardian; either she'd destroy the rock, or have her men slaughtered. With the Helvault destroyed, both Avacyn and the demons were unleashed back into the world.
In-Game References
Represented in cards:
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External information
http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/stf/178 The prison of Silver