The Lord … shall stir up a scourge for him; or, lift up a scourge over him. Isaiah uses the metaphor of the "scourge" again in Isaiah 28:16, Isaiah 28:18. It is rare in Scripture, though common among the Greek and Latin writers.
According to the slaughter of Midian at the rock of Oreb (comp. Isaiah 9:4). The "slaughter of Midian at the rock of Oreb" was that great destruction of the Midianites which was begun by the three hundred under Gideon, and completed by the men of Ephraim, whereof we have an account in 7:19-25.
Its counterpart in Assyrian history would seem to be the destruction of Sennacherib's army, as related in 2 Kings 19:35. As his rod was upon the sea. An allusion to the drowning of Pharaoh's host in the Red Sea.
This was a nearer parallel to the destruction of Sennacherib's army than the slaughter of the Midianites, since it was wholly miraculous. By "his rod" we may understand the rod of Moses, endued by God with miraculous powers (Exodus 4:3, Exodus 4:4; Exodus 14:16, Exodus 14:27).
After the manner of Egypt; i.e. "after the manner of his action in Egypt."