The word "he" refers to the enemy king and his army. The great size of the army is spoken of as if they were giant storm clouds. Alternate translation: "See, great armies like giant clouds are gathering to attack" or "See, great armies are gathering to attack" (See: [[rc://en/ta/man/translate/figs-simile]])
The enemies' chariots are spoken of as if they were like a storm. Storms can be noisy, destructive, and fast-moving. Alternate translation: "the roar of their chariots coming is like a fierce storm" (See: [[rc://en/ta/man/translate/figs-simile]])
This can be stated in active form. Alternate translation: "for they will devastate us" or "for they will destroy us all" (See: [[rc://en/ta/man/translate/figs-activepassive]])