Circe becomes extremely angry and jealous, not at Glaucus, but at Scylla. Instead of making a love potion for Glaucus, Circe makes a potion that will poison Scylla. She pours the poison into the bay where Scylla bathes, awaiting her arrival.
As Scylla enters the water, the poison turns her into a hideous monster, made of several heads with fierce dogs and ugly serpents.
For eternity, Scylla is rooted to that rock. Everything that is in her reach, she destroys in utter misery and hate. So, sailors are warned, for Scylla is a peril to all those who cross her.