MHH 18

By degrees we beheld the infinite Abyss, fiery as the smoke 
of a burning city; beneath us at an immense distance was the sun,
black but shining[;] round it were fiery tracks on which revolv'd
vast spiders, crawling after their prey; which flew or rather
swum in the infinite deep, in the most terrific shapes of animals
sprung from corruption. & the air was full of them, & seemd
composed of them; these are Devils. and are called Powers of the
air, I now asked my companion which was my eternal lot? he said,
between the black & white spiders 
  But now, from between the black & white spiders a cloud and
fire burst and rolled thro the deep blackning all beneath, so
that the nether deep grew black as a sea & rolled with a terrible
noise: beneath us was nothing now to be seen but a black tempest,
till looking east between the clouds & the waves, we saw a
cataract of blood mixed with fire and not many stones throw from
us appeard and sunk again the scaly fold of a monstrous serpent.
at last to the east, distant about three degrees appeard a fiery
crest above the waves slowly it reared like a ridge of golden
rocks till we discoverd two globes of crimson fire. from which
the sea fled away in clouds of smoke, and now we saw, it was the
head of Leviathan. his forehead was divided into streaks of green
& purple like those on a tygers forehead: soon we saw his mouth &
red gills hang just above the raging foam tinging the black deep
with beams of blood, advancing toward (Erdman 41)

Behemoth and Leviathan
Illustrations to the Book of Job

Job.40
[15] Behold now behemoth, which I made with thee; he eateth grass as an ox.
Job.41
[1] Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?
Isa.27
[1] In that day the LORD with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea.
[10] And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.

Erdman, at page 115 of The Illuminated Blake refers to the script as barren, by which I suppose he may have meant 'print-like' rather than the usual cursive; it suggests barren thoughts, no doubt with vivid meaning to Blake, but if I dreamed it, it could be nothing but a black nightmare.

Blake is hanging topsy-turvy head down into the 'infinite Abyss', which looked like a 'burning city' and far below a shining black sun (the title of many novels).  It was esoterically connected with the Nazi regime.

Around the sun white and black spiders revolve. They might represent black and white devils, the black ones fearful and the white ones crafty and pernicious.  Their prey swam like various attractive and hideous animals; "these are Devils and are called the Powers of the air" (See Ephesians 2:2):

"2 Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:"

The next paragraph might well be considered a reference to Revelations. For example Rev 8:8

"[8] And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood;"

The "monstrous serpent" also suggests Behemoth and Leviathan portrayed in Illustrations of the Book of Job:

These words also evoke Revelation 12:

[9] And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.