Avacyn is the Angel Protector of the Plane of Innistrad. She has protected the plane for many years from evil and demons and helps keep the vampire lords in check. The Order of Avacyn was founded in her name with its headquarters in Gavony. She went missing for a while and evil activity ran rampant.
The party freed her from the Helvault where she was mysteriously trapped in. The party doesn't really like her or understand why she has been so loved and worshipped.