Madge is in the middle of the ocean when a harbinger of dread reveals that after months of no word, Lilith is on the move and plans to bring her legions of siren-demons to their world.

An ancient grimoire, coveted by a hidden coven of witches in Norway, holds the key to stopping her. But Madge is unprepared for what comes with it; unleashing a power within her she must learn to wield if she's to call forth a behemoth from the deep that could destroy her, too.