Gilgamesh

"The god-king inherited the furry-man's savage strength and stout heart."

- Lore

A legend from Ancient Mesopotamia, the Epic of Gilgamesh is the one of the oldest known works of great literature, dating from around 2100-1200 BCE and inscribed on clay tablets.

Mesopotamian Mythology
The story follows the eponymous Gilgamesh, the Semi-Divine King of Uruk. Gilgamesh is far from an ideal king, spending most of his days partying, picking fights, and laying with people's wives, sometimes all at once. Understandably upset, the people pray for the gods to give their king a companion who can temper his wrath.

Their prayers are answered in the form of Enkidu, a Wild Man who emerges from the woods to fight Gilgamesh. The two brawl in the streets of Uruk until neither can continue, and by the end the two have become the greatest of friends. For a while, they entertain themselves with grand adventures, but eventually their actions anger the gods, who strike Enkidu down.

Heartbroken by the loss of his friend and suddenly aware of death's inevitability, Gilgamesh sets out on one last adventure in search of immortality. He ultimately fails, but in the process learns a valuable lesson about arrogance, and returns to rule Uruk as a wiser, humbler king.

Hades
Though Gilgamesh himself doesn't physically appear in the game, his legendary weapon the Twin Fists is seen and can be used by Zagreus, implying that he may've died at some point prior to the events of the game.

Additional notes
For additional information on Gilgamesh that does not pertain to Hades, see Wikipedia's article: Gilgamesh