Stheno was born in the earliest days of the cosmos, before the Olympian gods had won their throne, when the world was still populated by primordial terrors that sprang from the sea itself. Her father was Phorcys, the old man of the sea, a primordial deity half man and half fish with crab-claw forelegs and red spiked skin, who dwelled in a cave at the bottom of the ocean with his wife Ceto, the goddess of the sea's darkest depths. Together they produced a family of horrors: the Graeae, three grey sisters born old who share one eye and one tooth between them; Echidna, the half-woman half-serpent mother of the Chimera, Cerberus, and the Hydra; Ladon, the hundred-headed dragon who guards the golden apples of the Hesperides; and the three Gorgons themselves. Stheno was the eldest, born first and strongest, with a nature that was all aggression and no mercy. Her name means the mighty one or the forceful, and every source that mentions her agrees that she was the most terrible of the three sisters in battle, the one whose claws drew the most blood and whose gaze turned the most warriors to stone.

Hesiod, the poet who wrote the Theogony, the great genealogy of the gods, describes the Gorgons as winged women with round faces, large staring eyes, wide mouths, lolling tongues, the tusks of swine, flared nostrils, and serpents for hair. Their hands were brazen, their claws golden, and their feet swift. All three could turn any living creature to stone with a single look, but Stheno and Euryale possessed something their younger sister did not: immortality. They were ageless, deathless, beyond the reach of any blade or poison. Medusa was mortal, a fact that would make her the target of every hero who sought a Gorgon's head, while Stheno and Euryale remained forever beyond such quests. The poet Pindar, writing in the fifth century BCE, makes explicit what Hesiod only implies: all three sisters had horrible snakey hair, and all three were equally terrible to behold. But Stheno was the worst. Later traditions, recorded by the mythographers, state that Stheno killed more men than both her sisters combined, that her strength was absolute and her rage unquenchable, that she was the true monster of the trio while Medusa was merely the most famous.

The Gorgons lived far beyond the river Oceanus, the stream that encircles the entire world, on a rocky island called Sarpedon where night meets the western sun and the Hesperides, the nymphs of the evening, sing beside the golden tree. It was a place no mortal ship could reach, a fastness of stone and darkness where the sisters slept by day and hunted by night. Some later poets placed them in the deserts of Libya instead, where their gaze turned travellers to pillars of salt and their serpent hair coiled among the dunes. Wherever they dwelled, they were shunned by gods and men alike, a trio of terror that needed no kingdom and no worship, only the isolation of their own monstrous company.

The only story in which Stheno appears is the one that destroyed her family. Perseus, the demigod son of Zeus and the princess Danae, was sent by King Polydectes of Seriphos to fetch the head of a Gorgon. The gods equipped him with divine weapons: Athena gave him a polished bronze shield that shone like a mirror; Hermes gave him winged sandals and a curved sickle of adamant; Hades gave him the helmet of darkness that made the wearer invisible. Perseus flew across the western ocean, past the Hesperides and the pillars that hold the sky, until he reached the island of the Gorgons. He found all three sisters sleeping upon their rock in the midday heat, their serpent hair coiled upon the stone, their brazen claws relaxed, their terrible eyes closed. Stheno lay on one side of Medusa, Euryale on the other, and all three were at rest.

Perseus, guided by Athena, approached with his eyes fixed not on the monsters but on the reflection in his polished shield. He saw Medusa's image in the bronze, the terrible face made harmless by reflection, and with a single stroke of the adamant sickle he severed her head from her body. The moment the blade cut through her neck, two beings sprang from the wound: Pegasus, the winged horse, and Chrysaor, the golden warrior. The noise of the beheading woke Stheno and Euryale. They rose with shrieks that shook the island, their wings beating the air, their serpent hair hissing in fury, their brazen claws reaching for the killer of their sister. Stheno was first to rise, her immortal eyes blazing with a rage that had no limit, for she had lost the one sister who could die, the one sister who had been mortal and therefore vulnerable, the one sister she had protected without knowing it.

The two immortal Gorgons pursued Perseus across the sky. The Hesiodic Shield of Heracles describes the scene as it was depicted on the hero's shield: Perseus outstretched and shuddering as he fled, the Gorgons rushing after him with terrible speed, their girdles hung with serpents that arched forward and licked the air with their tongues, grinding their teeth with strength and glaring savagely, great Fear rioting upon their heads. But Perseus had already seized the severed head, placed it in the kibisis, a special wallet of woven silver and gold, and donned the helmet of Hades. He was invisible, a shadow in the wind, and though Stheno and Euryale pursued him across the western ocean, their claws grasping at empty air and their shrieks echoing across the waves, they could not find him. Pindar records that Athena, the goddess who had guided Perseus's hand, later invented the flute to imitate the shrill cry of Euryale's lament, but Stheno's voice was not music. It was pure fury, the roar of an immortal monster who had been robbed and could not be avenged.

Stheno and Euryale returned to their island. They had no bodies to bury, for Medusa's blood had already given birth to Pegasus and Chrysaor, and her head was carried away in the wallet of Perseus. They returned to the empty rock where their sister had slept, and there they remained, immortal and ageless, guarding the place where the mortal Gorgon had died. What Stheno did in the centuries that followed, the old stories do not say. She was immortal, beyond death and beyond change, and the world forgot her in favour of the sister who had been killed. But she never died. She never grew old. She never weakened. She remained on her island beyond Oceanus, the mighty one, the forceful, the Gorgon who could not be slain, waiting for any fool who might dare to approach her rock again.