Haman was hanged on the gallows he had prepared for whom?

Answer: Mordecai

Haman had built a gallows for Mordecai, but the king ordered Haman himself hanged on it.

“Then Harbonah, one of the eunuchs attending the king, said: “There is a gallows fifty cubits high at Haman’s house. He had it built for Mordecai, who gave the report that saved the king.” “Hang him on it!” declared the king. So they hanged Haman on the gallows he had prepared for Mordecai. Then the fury of the king subsided.”— Esther 7:9-10 (BSB)