A magical Obelisk in Gavony
Avacyn and her angels use this stone to imprison demons who do not die easily or can reincarnate.
Time passed by differently in this prison.
At the end of Session 1: The Masquerade, we learn that Avacynhad been trapped in here as well. Sorinand the team shattered the obelisk to free her which also unleashes the demons. (Get to work Avacyn!)
The Helvault was a huge rough, angular monolith made of silver rock that existed on Innistrad. It was created by Sorin and functioned as a prison for supernatural creatures. Stories say it and Avacynfirst appeared on the same night, and it remains the most holy object on Innistradafter Avacynherself.
A piece of Innistrad's silver moon, the Helvault stood at the precipice inside the courtyard of the High City of Thraben Cathedral, overlooking the lake beyond. Its surface was rough and unrefined, and thin veins of dark mortar branched across its surface.