In the complex mythology of William Blake, Urizen was the embodiment of reason and law.
He is usually depicted as a bearded old man; he sometimes bears architect's tools, to create and constrain the universe; or nets, with which he ensnares people in webs of law and conventional culture.
He gets much of his characterization from popular conceptions of Yahweh, the god of the Old Testament. The name may come from "You Reason", i.e., the accepted wisdom of the age; or from the Greek horizein, "to set limits." Not a benevolent character, Urizen oppresses Orc, who embodies revolutionary passion and creativity, and who serves as a suffering saviour figure. He is also an enemy of Luvah, the spirit of love.
Urizen has clear similarities with the creature called the Demiurge by Gnostic sects. The symbolism of Freemasonry is another possible source of Blake's imagery for Urizen.