Behold, I will send a blast - and he shall hear a rumor - The rumor was, that Tirhakah had invaded Assyria. The blast was that which slew one hundred and eighty-five thousand of them in one night, see 2Ki 19:35.
Cause him to fall by the sword - Alluding to his death by the hands of his two sons, at Nineveh. See 2Ki 19:35-37.