Glaucus was a mortal in Greek mythology, who became immortal by eating a magical herb and turned into a prophetic god of the sea.
It is uncertain who his parents were; the sources disagree and have him be a son of Polybus and Euboea, Anthedon and Alcyone, or Poseidon and a Naiad nymph. He was a fisherman, who one day came across a magical herb; he realised that this herb would bring dead fish back to life, so he decided to eat it himself. As a result, he became immortal but at the same time, he started getting fish-like features, like fins and a fish tail. So, he ended up living in the sea, where the major deities Oceanus and Tethys accepted him in their realm. They taught him the art of prophecy, who later became the teacher of Apollo. As a former fisherman, he became the protector of fishermen and gave advice to anyone who was lost in the sea.
When Glaucus met the nymph Scylla, he fell in love with her, but she did not like him. So, he asked the witch Circe to brew a love potion. However, Circe fell in love with Glaucus, only to receive his scorn instead. Enraged, Circe made a poison which she poured into the pool where Scylla bathed, transforming her into a monster.