Eurybia belongs to the firstborn brood of the deep. Hesiod sets her among the children that Pontus, the barren Sea, got by lying with Earth: great Thaumas and proud Phorcys, fair-cheeked Ceto, and Eurybia who has a heart of adamant within her — alongside their eldest, the truthful Nereus, the Old Man of the Sea. So she stands at the very roots of the waters, a personification of the sea's mastery and force among the elder sea-powers. Apollodorus gives the same descent in plain catalogue: to Sea and Earth were born Phorcus, Thaumas, Nereus, Eurybia, and Ceto.2Hesiod · Pseudo-Apollodorus — Theogony 233–239; Bibliotheca 1.2.6

Hesiod marks her out with a singular epithet. Where her sisters are 'fair-cheeked,' Eurybia is the bright goddess who keeps a heart of adamant — flint, unyielding — in her breast, a figure of the sea's hard and tireless strength rather than its beauty. She is a minor goddess who acts in no surviving narrative; her significance is genealogical and emblematic, the deep's force made a person and a mother of powers.3Hesiod — Theogony 239

Crossing from the line of the Sea into the line of the Titans, bright Eurybia was joined in love to Crius, son of Uranus and Gaia, and bore him three sons: great Astraeus, and Pallas, and Perses, whom Hesiod calls eminent among all men in wisdom. Through them her blood reaches into the heavens — for to Astraeus the Dawn bore the strong-hearted winds, Zephyrus and Boreas and the rest, and the stars that shine in the wide sky. Apollodorus records the same union, naming Eurybia daughter of Sea as the mother by Crius of Astraeus, Pallas, and Perses.4Hesiod · Pseudo-Apollodorus — Theogony 375–377; Bibliotheca 1.2.2