The book takes its name from the character Urizen in Blake’s mythology, who represents alienated reason as the source of oppression. The book describes Urizen as the “primeaval priest”, and describes how he became separated from the other Eternals to create his own alienated and enslaving realm of religious dogma. Los and Enitharmon create a space within Urizen’s fallen universe to give birth to their son Orc, the spirit of revolution and freedom.

In form, the book is a parody of the Book of Genesis, with Blake’s Urizen being more similar to the demiurge of the Gnostics than a benevolent creator.

(Summary from Wikipedia)