Hades is the firstborn son of the Titans Cronus and Rhea — first to be devoured, last to be freed. When Cronus, fearing the prophecy that his own children would unseat him, swallowed each newborn in turn, Hades and his elder siblings spent their childhood inside their father's body, until Zeus, hidden away on Crete, returned with a draught that forced Cronus to disgorge them.
After the Titanomachy — the ten-year war that broke the old order — the three sons of Cronus divided the cosmos by lot. To Zeus fell the bright sky; to Poseidon the gray sea; and to Hades the realm beneath the earth: a kingdom no light visits, where every mortal soul will, in time, come down to him.
“We are three brothers… and all things were divided into three parts, and each was given his own domain. To me fell the gray sea forever; to Hades fell the misty darkness.”
Homer, Iliad XV.187–193
Hesiod records the same descent in his cosmogony: Cronus, son of Heaven, supplanted in his turn by his own children [1]. From this division, the Greek world ordered itself — sky above, sea between, and beneath them both the silent dominion of Hades.
[1] Hesiod, Theogony 453–506.