Esther 3:10

3:10 So the king removed his signet ring 1  from his hand and gave it to Haman the son of Hammedatha, the Agagite, who was hostile toward the Jews.

Esther 8:8

8:8 Now you write in the king’s name whatever in your opinion is appropriate concerning the Jews and seal it with the king’s signet ring. Any decree that is written in the king’s name and sealed with the king’s signet ring cannot be rescinded.

1 sn Possessing the king’s signet ring would enable Haman to act with full royal authority. The king’s ring would be used to impress the royal seal on edicts, making them as binding as if the king himself had enacted them.