In Book 1, Milton uses allusions and simile to convey Satan's enormous stature and power:
Thus Satan talking to his nearest mate
With head up-lift above the wave, and eyes
That sparkling blaz'd, his other parts besides
Prone on the flood, extended long and large
Lay floating many a rood, in bulk as huge
As whom the fables name of monstrous size,
Titanian, or Earth-born, that warr'd on Jove,
Briareos or Typhon, whom the Den
By ancient Tarsus held, or that sea-beast
Leviathan, which God of all his works
Created hugest that swim th' ocean stream...
Milton is referring to monsters and villains from Greek and Roman mythology: Titans (Titanian) and Giants (Briareos or Typhon) confined to hell (Tarsus) after rebelling against Jove (the king of the gods)—both portrayed as enormous, violent beings; the Leviathan is a Biblical sea-creature described in the Book of Job.
By comparing Satan to these notorious creatures—all of whom defied the gods or God, just as Satan has defied God—Milton is emphasizing the gravity of Satan's decision, while also carving out a place for Satan in a canon of mythological figures: insisting on his own power and notoriety, even though he has been banished to Hell. (Arguably, his banishment has only augmented his power.)
In Book 4, Milton describes the good angels preparing to battle Satan after he has been discovered in Paradise, and employs an allusion and a simile to describe them:
While thus he spake, th’ angelic squadron bright
Turned fiery red, sharp’ning in moonèd horns
Their phalanx, and began to hem him round
With ported spears, as thick as when a field
Of Ceres ripe for harvest waving bends
Her bearded grove of ears, which way the wind
Sways them; the careful ploughman doubting stands
Lest on the threshing floor his hopeful sheaves
Prove chaff.
Milton is comparing the throng of good angels to a field of corn or wheat (Ceres is the Roman god of agriculture, often represented with corn/wheat)—a fairly flimsy crop that is easily buffeted by the wind. Thus, the throng of angels is not really "thick" at all—but insubstantial and weak.
This may seem somewhat confusing, since Milton seems to implicitly support the good angels over Satan—yet they appear easily bested by him. But this also emphasizes the power of Satan's rage and rebelliousness (and the power of evil and corrupt authority in general): ultimately, the good angels are only able to defeat Satan after he has carried out his plot against Adam and Eve and thinks himself triumphant.
In Book 9, Milton uses a simile to liken Satan to a "black mist low creeping" before he transforms into a serpent to bewitch Eve:
So saying, through each thicket dank or dry,
Like a black mist low creeping, he held on
His midnight search, where soonest he might find
The serpent...
The "black mist" is a supernatural, seemingly unrealistic image. In real life, mist is colorless, and cannot actually be black. Yet the comparison also intuitively makes sense: fog at night, shot through with darkness, can resemble a "black mist." Thus, this simile highlights Satan's double-edged slipperiness. Once Satan assumes the serpent's form, he will pass as an animal—an ordinary part of Eden's natural landscape—while also appearing otherworldly and strange. For example, his weaving motions as he leads Eve to the Tree of Knowledge are so mesmerizingly complex that they make "intricate seem straight." Eve will be drawn to Satan in his serpent form for these very reasons: he will appear ordinary enough not to arouse her suspicions, but his entrancing appearance will lead her to temptation.
The "black mist" also suggests creeping contagion and invasion. Just as mist silently spreads out over a landscape, Satan is quietly infiltrating Eden—after being expelled during his first trip in Book 4—during "his midnight search" for a serpent's body to inhabit.
In Book 11, the archangel Michael ushers Adam and Eve out of Paradise and shows Adam visions of future events to come—including striking imagery that tells the story of Noah's Ark (in which a simile is contained):
Meanwhile the south wind rose, and with black wings
Wide hovering, all the clouds together drove
From under heav’n; the hills to their supply
Vapour, and exhalation dusk and moist,
Sent up amain; and now the thickened sky
Like a dark ceiling stood; down rushed the rain
Impetuous, and continued till the earth
No more was seen; the floating vessel swum
Uplifted; and secure with beakèd prow
Rode tilting o’er the waves, all dwellings else
Flood overwhelmed, and them with all their pomp
Deep under water rolled; sea covered sea,
Sea without shore; and in their palaces
Where luxury late reigned, sea-monsters whelped
And stabled; of mankind, so numerous late,
All left, in one small bottom swum embarked.
This intricate description of the great flood (sent by God as a punishment against postlapsarian humans—Noah's ark is the "floating vessel" that survives) makes the consequences and significance of Adam and Eve's disobedience starkly apparent, with phrases that render vivid the scale of the flood—"sea cove[ring] sea," "impetuous" rain, the earth "No more was seen" in the midst of the flooding.
By comparing the "thickened sky" to a "dark ceiling," Milton (describing the image Michael has conjured) is indicating God's absolute authority over humankind: Heaven, in the sky, is the "ceiling" under which mortals live—there is no tier beyond it—and God has the capacity to destroy their world.
Milton's imagery allows the reader to stand in Adam's place: the reader is transported, as Adam has been, to a scene of terror and destruction—which helps the reader to empathize with Adam while also understanding (in a visceral way) the magnitude of God's power.