A Revivification Ritual was a dark magic ritual intended to resurrect a vampire. It was implied that the ritual has been performed on other occasions, but it had never been successful. Absalom, alongside Ned, Bob, Jane and Walt, attempted to perform this kind of ritual to resurrect The Master.
The ritual required any and all humans who were present when the vampire died. The humans would be hung upside down over the dead vampire's remains, and their throats would be cut. The blood would run down onto the vampire's remains, bringing it back to life.