Esther became queen to the king of which empire?

Answer: Persia

Xerxes ruled Persia and Media from the citadel of Susa, and he set the royal crown on Esther's head.

“In the third year of his reign, Xerxes held a feast for all his officials and servants. The military leaders of Persia and Media were there, along with the nobles and princes of the provinces.”— Esther 1:3 (BSB)