Grendel, also identified as Queen Auriana in certain versions, is the primary antagonist. She rules a dark realm from her castle, systematically capturing children. These victims are transformed into harpies via a specialized machine, swelling the ranks of her enslaved forces. Failure results in exile to the Cave of Eternal Ice, trapping the offender in perpetual ice.

Her entrapment strategy employs deceptive lures, including a dwelling designed like confectionery, echoing folkloric witch tactics. Her specific method focuses on aerial capture using minions such as Allegra. A crystal ball fuels her magic, granting surveillance, spellcasting, and control over transformations. Destruction of this artifact severs her power, stripping away her disguise to reveal a grotesque, goblin-like true form.

Her reign ends when a time bomb affixed to her escape balloon detonates. During a frantic attempt to dislodge the explosive, the balloon collides with the castle and detonates with catastrophic force, killing her. This event shatters all curses, reverting every harpy to their original human form.