The Helvault is a huge rough, angular monolith made of silver rock that existed on Innistrad. It was created by Sorin Markov and functioned as a prison for supernatural creatures. Stories say it and Avacyn first appeared on the same night, and it remains the most holy object on Innistrad after Avacyn herself.
A golden helix streaked skyward from the Helvault. A thunderous explosion shattered the silver monolith and Avacyn emerged, free from her prison at last.
A piece of Innistrad’s silver moon, the Helvault stood at the precipice inside the courtyard of the Thraben Cathedral, overlooking the lake beyond. Its surface was rough and unrefined, and thin veins of dark mortar branched across its surface. The Lithomancer Nahiri while able to control stone, was not able to influence the Helvault.
Now a planeswalker, when Sorin returned to Innistrad, he saw the threat to life that vampires represented, and realising that vampires couldn’t exist without humans, he decided to take action: he created the archangel Avacyn to safeguard humanity, providing both vampires and humans a way to subsist in the world. But the angel was only half of the protective measures Sorin created to protect Innistrad. The other half was the Helvault. It came from the plane’s silver moon, and it had taken great effort for Sorin to bring it down.
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.»
In Dungeons & Dragons
As explained in Tasha’s Cauldron of Everything, not all lands thrive as nature intended. Magical forces, strange interlopers, or tragic events can alter an area’s destiny, fundamentally changing the land’s innate character, taking on new qualities.
The Helvault turns the area surrounding it into a a supernatural region permeated by the grace of the angel Avacyn and the angelic flights supporting her. While inside the area of influence of the Helvault, a creature has advantage on saving throws compelled by the abilities of a fiend, undead or shapeshifter. Similar creatures make saving throws at disadvantage.
Additionally, the area allows followers of Avacyn to cast banishment as a ritual with an additional effect: its duration is permanent within the demiplane inside the Helvault. This version of the spell requires the use of a special material component: a collar made of silver worth at least 100gp and forged in the shape of Avacyn’s symbol. Finally, if a weapons is forged or enchanted in the area during the Feast of Goldnight (which are two days in a row happening once a year during the Harvest Moon) they are considered silvered for purposes of overcoming damage reduction.
Given its main values as protectors of humankind and nemesis of vampires, werewolves and demons, the best way to place the Church of Avacyn in Eberron is as an offspring or heretic sect within the Church of the Silver Flame. When I reimagined the Innistrad storyline for one of my campaigns («Atur: Sins and Portents«) I did it so in Karrnath’s countryside. This allowed me to have some distance from Thrane and most civilization for this sect to go unnoticed by ecclesial authorities.
Are you in the hunt for demons, lycanthropes and vampires? Are you planning to research and locate the famous Helvault and take advantage of its properties? Also… do you know how Avacyn ended? You may want to find out.