Isaiah said the treasures Hezekiah showed the envoys would be carried off where?

Answer: To Babylon

Isaiah declared that everything in Hezekiah's palace would one day be carried off to Babylon, with nothing left.

“Then Isaiah said to Hezekiah, “Hear the word of the LORD: The time will surely come when everything in your palace and all that your fathers have stored up until this day will be carried off to Babylon. Nothing will be left, says the LORD.”— 2 Kings 20:16-17 (BSB)