The Book of Urizen might well be thought of as a sort of Introduction or Preface to The Four Zoas. Here we meet zoas, who will be fleshed out with tremendous detail in the major prophesies (The Four Zoas, Milton, and Jerusalem). There are 27 plates in all, and each of them might provide a source for further study and understanding of Urizen and of Blake's myth as a whole.
At the final consummation Blake rehabilitated Bacon, Newton and Locke. They appeared counterbalancing Blake's three great poets.
rejoicing terrific vanishing
Fourfold Annihilation & at the clangor of the
Arrows of Intellect
The innumerable Chariots of the Almighty appeard
And Bacon & Newton & Locke, & Milton &
Shakspear & Chaucer"
(Jerusalem 98: 6-9 )
In Night II of The Four Zoas Urizen lost his faith and
in vision saw the world collapsing into darkness:
"Urizen rose from the bright Feast like a star thro' the evening sky
Exulting at the voice that calld him from the Feast of envy
First he beheld the body of Man pale, cold, the horrors of death
Beneath his feet shot thro' him as he stood in he Human Brain
And all its golden porches grew pale with his sickening light
No more Exulting for he saw Eternal Death beneath
Pale he beheld futurity; pale he beheld the Abyss...[he said]
Build we a Bower for heavens darling in the grizly deep
Build we the Mundane Shell around the Rock of Albion"
FZ2: 23:9-24.8; (E 314)
The darkness led Urizen to this confession:
Was a reflection of his face who calld me from the deep?
I well remember for I heard the mild & holy voice saying
'O light spring up and shine' and I sprang up from the deep
He gave to me a silver scepter & crownd me with a golden crown and said
Go forth & guide my Son who wanders on the ocean.
I went not forth. I hid myself in black clouds of my wrath
I calld the stars around my feet in the night of councils dark
The stars threw down their spears and fled naked away.
(Four Zoas Night 5 64:20-28, 
Urizen symbolized the demiurge, used by gnostic and other philosophers as the lesser god who created this vale of tears, our world. Urizen often shaded into a Moses like figure, constantly looking at and working on his books of the law, the vengeful Old Testament God, and other uncomplimentary names. In the fallen condition he shades into Satan:
"Urizen calld together the Synagogue of Satan in dire Sanhedrin
To Judge the Lamb of God to Death as a murderer & robber."
"Do I sleep amidst danger to Friends! O my Cities & Counties
Do you sleep! rouze up! rouze up. Eternal Death is abroad
So Albion spoke & threw himself into the Furnaces of affliction
All was a Vision, all a Dream: the Furnaces became Fountains of Living Waters How[l]ng from the Humanity Divine"
"And all the Cities of Albion rose from their Slumbers, and All
The Sons & Daughters of Albion on soft clouds Waking from Sleep
Soon all around remote the Heavens burnt with flaming fires
And Urizen & Luvah & Tharmas & Urthona arose into
Albions Bosom: Then Albion stood before Jesus in the Clouds
Of Heaven Fourfold among the Visions of God in Eternity"
(Jerusalem plate 96: near the end; Erdman 256)