You didn't need to give that much info for me to recognize it.
It was the Oracle of Delphi. Croesus wanted to attack Persia, and asked "her" for "advice"(*). Her response was, "If you attack, a mighty kingdom will be destroyed."
Unfortunately for Croesus, it was
his kingdom that was destroyed.
It reminds me of the Oracle of Proehl, who said "Tonight, a dynasty is born."
(*)Apparently, as near as I can tell from what I've read, the Oracle was a woman who got high off fumes emitted in/near the temple, and then made bizarre pronouncements that were left to temple attendants to "decipher."