Nereus is a Great Elder who tried to prevent Perenelle Flamel from leaving Alcatraz. He was worshipped by the ancient Greeks as the original Old Man of the Sea. In The Sorceress, Perenelle mentions that the last time he visited the human world was in in the year 1912; as the Titanic sank in the April of that year, this suggests that he may have been involved in that tragedy. He was killed by Black Hawk in The Enchantress.
The Old Man of the Sea as seen by Perenelle Flamel on Alcatraz in The Sorceress is a stocky, surprisingly-normal looking man with a head of thick curly hair flowing to his shoulders. He also has webbed fingers and a short beard twisted into two tight curls. Nereus wears a sleeveless jerkin of overlapping kelp leaves and strands of green seaweed and is accompanied by the stench of rotting fish. In his left hand he bears a wickedly spiked stone trident. He has no lower limbs; instead, he has eight suckered, octopus-like, tentacles.
- Immortality: Nereus is Immortal and does not age. Though an immortal may be poisoned or slain in battle, they may live for thousands or millions of years.