Albert Barnes Commentary Esther 2:21

"In those days, while Mordecai was sitting in the king`s gate, two of the king`s chamberlains, Bigthan and Teresh, of those that kept the threshold, were wroth, and sought to lay hands on the king Ahasuerus." — Esther 2:21 (ASV)

Conspiracies inside the palace were ordinary occurrences in Persia. Xerxes was ultimately murdered by Artabanus, the captain of the guard, and Aspamitras, a chamberlain and eunuch.