We've all heard someone dead pan: "I've got some good news and some bad news; which do you want to hear first?"
Isaiah's first short word from the LORD to Hezekiah followed the good news bad news pattern. See 2 Kings 19:7.
The good news for Hezekiah:"I am going to put such a spirit in him [Sennacherib] that when he hears a certain report, he will return to his own country [Assyria]." In other words, God will sovereignly turn Sennacherib's attention elsewhere so that Sennacherib will not destroy Jerusalem.
The bad news for Sennacherib: "I will have him [Sennacherib] cut down with the sword." In other words, not only will the LORD turn Sennacherib's attention away from Jerusalem, but the LORD will bring him down.
Isaiah will have more to say later, but for now, this will have to be sufficient to encourage Hezekiah.