In Greek mythology, Aegyptus (Aigyptos) was a legendary king whose name was folk-etymologized onto Egypt. Twin brother of Danaus and son of Belus, he fathered fifty sons whom he sought to marry to Danaus' fifty daughters, the Danaïdes. After the wedding-night massacre, only Lynceus survived with Hypermnestra and founded a new Argive line.
Adapted from Wikipedia, "Aegyptus" · CC BY-SA 4.0
I.Belus' twins and fifty sons
Aegyptus was son of Belus (often by Achiroe, a Nile naiad, or by Sida) and twin of Danaus, king of Libya; Euripides sometimes adds Cepheus and Phineus as further brothers. Through Belus he descends from Io. Apollodorus lists his fifty sons by several wives; other authors give a single mother.
One tradition has him rule Arabia, conquer the Melampodes, and rename that land Egypt after himself. A scholium reverses the geography, placing the twins first in Argolis before Aegyptus flees to the land that takes his name.
II.The Danaïdes
When Aegyptus demanded that his sons marry Danaus' daughters, Danaus fled with the girls to Argos. Pressed there, he consented to the marriages but ordered his daughters to kill their husbands on the wedding night. Forty-nine obeyed; Hypermnestra spared Lynceus, who later avenged his brothers in some accounts.
The flight and plea of the Danaïdes are the subject of Aeschylus' Suppliants. Later moralizing tradition punishes the murderesses in the underworld with a leaking sieve or jar—an image of endless, futile labor.