Milton writes in a very elevated, allusive, and dense style. If we had to pick one word to sum up his style that word would be Latinate. Latinate means characteristic of the Latin language (a "dead" language used in Virgil's Aeneid and father of modern Romance languages like Spanish, French, and Italian). In Latin, word order doesn't matter (we could tell you why but it would take forever), which allows for some very cleverly structured poetry. Milton, being a lover of classical languages, attempts to emulate Virgil's style in particular, often leaving words out (and thus expecting the reader to supply them), using a funky word order (verbs are often placed in strange places), using words in older senses that play upon the word's roots (Milton refers to Satan's "ruin," playing on the Latin root ruere, to fall) and the like.
Milton writes in Book 5: "Deep malice thence conceiving and disdain" (666). What he means is that they were "conceiving" "deep malice" and "disdain." However Milton sandwiches the participle (a verbal form ending in "ing") "conceiving" in between its two objects, "deep malice" and "disdain." As another example, take the very first sentence of the poem (which is sixteen lines longs!). There, he delays the main verb for nearly six lines. What Milton means is "Sing Muse of man's first disobedience, and the fruit of that Forbidden Tree," but he inverts the order and starts with "Of man's first disobedience, and the Fruit/ Of that Forbidden Tree […]," finally arriving at "sing" in line 6.