The only way I know to get any grasp of this picture is by reading George MacDonald's Lillith which you can do online at this site. MacDonald takes one into a cavern where the dead sleep until they are healed and awake to the reality Blake calls Eternity. Like Blake, MacDonald was interested in revealing the world which is real and spiritual to those who perceive only the false and material. Both men created myths which reveal truth which cannot be contained in the forms of logic and rationality.
Descent into Death
In the image which appears as the frontispiece of Jerusalem is shown Los entering a door leading down into a dark space which he must explore to restore Albion to wholeness. The present image could be an elaboration on Blake's image for Robert Blair's The Grave which is named The Soul exploring the recesses of the Grave.
Notice that Blake portrays the individual in multiple statuses simultaneously. The individual in a natural body is exploring or observing; as spiritual body or soul, the individual also illumines and guides; the individuals who has entered death are in repose within the various caverns.
The shifting meanings of death, the grave, sleep, awakening and annihilation are seen in this passage from Milton. The image of descending into the recesses of the grave, and poetry from Milton complement one another and shed light for understanding both.
Milton, Plate 14 , (E 108)
"Then Milton rose up from the heavens of Albion ardorous!
The whole Assembly wept prophetic, seeing in Miltons face
And in his lineaments divine the shades of Death & Ulro
He took off the robe of the promise, & ungirded himself from the
oath of God
And Milton said, I go to Eternal Death! The Nations still
Follow after the detestable Gods of Priam; in pomp
Of warlike selfhood, contradicting and blaspheming.
When will the Resurrection come; to deliver the sleeping body
From corruptibility: O when Lord Jesus wilt thou come?
Tarry no longer; for my soul lies at the gates of death.
I will arise and look forth for the morning of the grave.
I will go down to the sepulcher to see if morning breaks!
I will go down to self annihilation and eternal death,
Lest the Last Judgment come & find me unannihilate
And I be siez'd & giv'n into the hands of my own Selfhood
The Lamb of God is seen thro' mists & shadows, hov'ring
Over the sepulchers in clouds of Jehovah & winds of Elohim
A disk of blood, distant; & heav'ns & earth's roll dark between
What do I here before the Judgment? without my Emanation?
With the daughters of memory, & not with the daughters of
I in my Selfhood am that Satan: I am that Evil One!
He is my Spectre! in my obedience to loose him from my Hells
To claim the Hells, my Furnaces, I go to Eternal Death."