Scylla

Once a sea nymph of great beauty, Scylla was transformed by Circe (in jealousy over Glaucus's love) or by Amphitrite (jealous of Poseidon's attentions). She was turned into a monster with twelve tentacle-like legs, six heads on long necks, and three rows of teeth in each mouth. She haunted a rock in the Strait of Messina opposite the whirlpool Charybdis.

From the waist up, a woman; below that, twelve tentacle-legs and six long serpentine necks, each ending in a head with three rows of razor-sharp teeth. She was anchored to her rock, perpetually reaching out to snatch passing sailors.

Each of her six heads could snatch a sailor from a ship passing below. She was effectively unkillable — even Heracles is said to have killed her, only for her father Phorcys to revive her. Her position forced navigators to sail close enough to Charybdis or lose crew to her.