Belus was born from the union of Poseidon, the earth-shaking god of the sea, and Libya, the daughter of Epaphus and Memphis. Epaphus was himself the son of Io, the priestess of Hera whom Zeus had loved and transformed into a heifer, sending her wandering across Europe and Asia until she reached Egypt and gave birth to a son by the Nile. That son, Epaphus, married Memphis, a nymph of the river, and their daughter Libya gave her name to the whole northern coast of Africa. Poseidon saw Libya and desired her, and from their union came twin sons: Agenor and Belus. The brothers were destined to rule the two halves of the Mediterranean world. Agenor departed for Phoenicia, where he founded the cities of Tyre and Sidon and became the father of Europa, Cadmus, Cilix, and Phoenix. Belus remained in Africa, inheriting the kingdom of his grandfather Epaphus, and ruled as king over Egypt and the lands that bore his mother's name.

Belus married Achiroe, whom some call Anchinoe, a daughter of Nilus, the river god whose waters made Egypt the gift of the Nile. By her he fathered at least two sons, and in some accounts many more. The most famous were Aegyptus and Danaus, twin brothers who would become the protagonists of one of Greek mythology's darkest stories. Aegyptus was given Arabia to rule, and Danaus was given Libya. But Aegyptus was not satisfied. He conquered the land of the Melampods, a territory that probably lay within Danaus' own kingdom, and named the conquered country after himself. From that day, the land was called Egypt, and Aegyptus ruled it as his own. Danaus, seeing his brother's power grow, grew fearful. The rivalry between the twins festered until it became intolerable.

Aegyptus fathered fifty sons. Danaus fathered fifty daughters, the Danaids. Aegyptus decided that his fifty sons would marry his brother's fifty daughters, binding the two houses together in a union that would end the feud. Danaus saw the proposal for what it was: a plot to absorb his line into his brother's and eliminate his independence. He refused. Aegyptus pressed the matter, and Danaus, in desperation, fled. He built a ship - the first vessel ever constructed, according to some traditions - and sailed with his fifty daughters across the Mediterranean to Argos in Greece. There he claimed the throne from the descendants of Io, his own great-great-grandmother, and established himself as king. But Aegyptus' sons were not deterred. They pursued their cousins across the sea, landed in Argos, and demanded the marriages that their father had arranged. Danaus, unable to refuse outright without starting a war he could not win, agreed. On the wedding night, however, he gave each of his daughters a dagger and commanded them to kill their bridegrooms. Forty-nine of the Danaids obeyed. Only Hypermnestra, the eldest, spared her husband Lynceus, because he had respected her virginity. The forty-nine heads were buried, and the forty-nine murderesses were condemned in the underworld to carry water in leaky jars for eternity. Belus, the grandfather of all hundred children, had set in motion a tragedy that would stain his name forever.

But Belus was more than a genealogical node. His very name carried divine resonance. In the ancient Near East, the title Baal meant "lord," and it was applied to storm-gods across Canaan, Phoenicia, and Syria. The Greeks heard this name and incorporated it into their own mythic geography, making Belus a king who stood at the crossroads of Egypt and Phoenicia, of the Nile and the Mediterranean. The late epic poet Nonnus, writing in the fifth century CE, called him "the Libyan Zeus," a title that suggests the Greeks saw in him not merely a mortal king but a local manifestation of the thunder-god himself. Diodorus Siculus, the historian who wrote in the first century BCE, claimed that Belus founded a colony on the river Euphrates and appointed the Chaldean priests-astrologers whom the Babylonians revered, making him a bridge between Egyptian and Mesopotamian civilisation. Pausanias, the Greek traveller, recorded that Belus founded a temple of Heracles in Babylon, a claim that links the Greek hero-cult to the ancient East.

The family of Belus extended far beyond the Danaids. According to Pherecydes of Athens, an early Greek mythographer, Belus had a daughter named Damno who married her uncle Agenor and bore him Phoenix and two daughters, Isaie and Melia, who in turn married their cousins Aegyptus and Danaus. The Hesiodic Catalogue of Women made Belus the father of a daughter named Thronia, on whom Hermes fathered Arabus, the eponym of Arabia. Some sources made him the father of Lamia, the Libyan queen who became a child-eating demon after Hera killed her children. Antoninus Liberalis named him as the father of Thias, who in turn fathered Smyrna, the woman who conceived a passion for her own father and was transformed into a myrrh tree. Euripides added Cepheus and Phineus to the list of Belus' sons, and Nonnus gave him five: Phineus, Phoenix, Agenor, Aegyptus, and Danaus, though Nonnus elsewhere confused the generations and made Phineus a brother of Cadmus. The ancient sources disagree on almost every detail of Belus' family, but they agree on one thing: he was the root from which the royal houses of Egypt, Phoenicia, and Greece all claimed descent.