The Talmud (TB Megillah 13b) states that only Mordecai understood Bigthan and
Teresh, who conversed in their native Tarsian when plotting against the king.
For example, Mordecai "accidentally" witnessed the conversation between Bigthan and
Teresh at the king's gate (Esth 2:19-22) and uncovered a plot to kill the king, which had to be rewarded by Ahasuerus.
Bigtan and
Teresh, two palace guards were arrested recently on charges of treason for plotting to assassinate King Achashverosh.
Mordechai, for example, reports on Bigtan and
Teresh's plot to assassinate Ahasuerus "in the accents of the Megillah" (33); Vashti, being led to execution, asks to be spoken kindly of "when you play the Purim play" (40), and Haman, in his uneasy dreams, sees Mordechai gloating above him ...