The Innistrad storyline takes place on the plane of the same name and revolves around the creation of a balance of power by the archangel Avacyn so the humans of Innistrad could survive. Shortly before the events of the Innistrad set Avacyn was challenged to a battle by the demon Griselbrand who had landed on the Helvault in an act of defiance.
The battle took place out of the view of all but Avacyn’s host, Mikaeus the Lunarch and the highest ranking clergymen in the Church of Avacyn. At the climax of the battle Avacyn decided to trap Griselbrand within the Helvault and began casting the binding spell. Before she could finish the spell the demon impaled her through the heart with her own spear causing the spell to backlash and Avacyn to be pulled into the Helvault with Griselbrand.
During the course of Innistrad set we learned about how Avacyn’s disappearance caused the protective wards and the faith based magic her followers employed to become increasingly weaker as time progressed.
Then in the Dark Ascension set we learn about the creation of vampirism on Innistrad, the creation of Avacyn and how the balance of power on Innistrad came to be. We also learn about the siblings Geralf and Gisa and how they exploited the weakened state of the Church of Avacyn and its wards and launched their siege of Thraben in the hopes that Geralf could kill the Lunarch Mikaeus and take his place.
In the final set of the block, Avacyn Restored, Liliana Vess completes her quest to find the location of the demon Griselbrand. Using Thalia, the Guardian of Thraben, as tool Liliana gave her an ultimatum. Either she will destroy the Helvault or her men will be killed. Thalia destroyed the Helvault and saved her men and unknowingly released the archangel Avacyn, the demon Griselbrand along with all of the other demons who had been imprisoned within the Helvault by Avacyn over the years.
In the Innistrad’s closing chapters we learn that Liliana tracked down the demon Griselbrand and slew him. We also learn that Avacyn’s return caused a surge in the holy magic which allowed the Church of Avacyn to lead an offensive campaign against the vampires, werewolves, skaab, demons, devils and other evils that plagued humanity in Avacyn’s absence. Shortly after her release Avacyn cast The Cursemute which allowed the werewolves who wanted to be freed of their curse to be transformed into the Wolfir.