Esther 5:14

5:14 Haman’s wife Zeresh and all his friends said to him, “Have a gallows seventy-five feet high built, and in the morning tell the king that Mordecai should be hanged on it. Then go with the king to the banquet contented.”

It seemed like a good idea to Haman, so he had the gallows built.

Esther 9:25

9:25 But when the matter came to the king’s attention, the king gave written orders that Haman’s evil intentions that he had devised against the Jews should fall on his own head. He and his sons were hanged on the gallows.