Phorcys was born in the first generation of divine beings that followed the creation of the cosmos. His mother was Gaia, the broad-breasted Earth who had emerged from Chaos at the beginning of time, and his father was Pontus, the Sea, whom Gaia had produced without a mate in the earliest days of the world. Pontus was not the Mediterranean or the Aegean but the sea itself, the vast and trackless water that surrounded the known world, and his children inherited his ancient and alien nature. Phorcys had several siblings: Nereus, the old man of the sea whose fifty daughters the Nereids were beautiful and gentle, Thaumas, the father of Iris the rainbow and the Harpies, and Eurybia, who had power over the winds and the waves. But while Nereus and his daughters represented the calmer, more navigable aspects of the sea, Phorcys and his line embodied its depths, its darkness, and its capacity to produce things that should not exist.

He took his sister Ceto as his wife, a marriage that was common among the first generation of gods, who had no other peers to wed. Ceto was as ancient and terrible as he was, her name connected to the great sea-monsters that the Greeks called ketea, the vast creatures of the deep that could swallow ships whole. Together they began to produce children, and the children they produced were not beautiful like the Nereids or helpful like the river-gods. They were monsters, each one a blending of human and animal, mortal and divine, beauty and horror, as if the deepest parts of the sea could not create life without deforming it. Their first children were the Graeae, the three grey sisters named Deino, Enyo, and Pemphredo, who were born old and grey and who shared between them a single eye and a single tooth. They lived in a cave in the far west, beyond the sunset, and they were the keepers of secret knowledge, though their shared eye made them vulnerable to any thief bold enough to snatch it.

Their next children were the Gorgons, and these were worse. Stheno and Euryale were immortal, but the third, Medusa, was mortal, a distinction that would prove fatal. The Gorgons were born with snaky hair, bronze hands, and faces so terrible that anyone who looked directly upon them was turned to stone in an instant. They lived in the western ocean, near the boundary of the underworld, and their presence made the very islands around them uninhabitable. Phorcys and Ceto did not raise these children in any human sense; they were simply born into the world, and the world had to contend with them. The poet Hesiod, writing in his Theogony around 700 BCE, listed them among the children of Phorcys with the same matter-of-fact tone he used for rivers and mountains, as if the birth of monsters was simply part of the natural order, no more remarkable than the birth of a spring.

But Phorcys and Ceto were not finished. Their union also produced Echidna, a creature who was half beautiful woman and half monstrous serpent, who lived in a cave beneath a hollow rock far from both gods and mortals. Echidna would become the mother of the most famous monsters in Greek mythology through her union with Typhon, the hundred-headed storm giant. From her came Orthus, the two-headed hound of Geryon; Cerberus, the fifty-headed hound of Hades who guarded the entrance to the underworld; the Lernaean Hydra, the serpent whose heads grew back when cut off; the Chimera, the fire-breathing creature with the head of a lion, the body of a goat, and the tail of a serpent; the Sphinx who plagued Thebes with her riddles; and the Nemean Lion whose hide was impenetrable to any weapon. Phorcys was not merely the father of the Gorgons and the Graeae; he was the grandfather of nearly every monster that a Greek hero ever faced, the primordial ancestor of the entire bestiary of Greek myth.

Other children followed, each one adding to the catalogue of terrors that came from the deep. Phorcys and Ceto were also the parents of the Sirens, the creatures whose singing lured sailors to their deaths on rocky shores, though some said the Sirens were the daughters of the river-god Achelous and the muse Terpsichore. They were the parents of Scylla, the six-headed sea-monster who devoured sailors from her cave across from the whirlpool Charybdis, though other traditions gave Scylla different parents. And Phorcys alone, without Ceto, was the father of Thoosa, a sea-nymph whose union with Poseidon, the god of the sea, produced Polyphemus, the one-eyed Cyclops who trapped Odysseus and his men in his cave on the island of the Cyclopes and ate several of them before the hero blinded him with a sharpened stake. In this way, Phorcys's bloodline reached even into the Odyssey, the greatest adventure story in Greek literature, and the monsters that sprang from his line became the obstacles that defined heroism itself.

Phorcys himself was rarely described in action. Unlike Poseidon, who strode through the waves with his trident and stirred up storms, or Nereus, who was famous for his wisdom and his shape-shifting, Phorcys seems to have been a more static figure, a god of the deep places who produced monsters but did not himself roam the world. In art, he was sometimes shown as an old man with crab-like features, red skin, or a fish's tail, a visual representation of his connection to the strange creatures of the ocean floor. The harbour named after him on Ithaca, described by Homer in the Odyssey, was a natural inlet with two steep headlands that curved inward like arms, breaking the force of the wind and the waves and creating a sheltered anchorage. It was there that the Phaeacians, the seafaring people who had carried Odysseus home from their distant island, left him sleeping on the shore with his treasure, and it was from there that Odysseus woke and made his way to the palace he had not seen in twenty years. The fact that the hero's homecoming was staged in a harbour named for the father of monsters is one of those details that makes Greek myth so layered: the man who had escaped the Cyclops, the Sirens, and Scylla returned to a place that bore the name of the primordial source of them all.