Albert Barnes Commentary Judges 3:20

"And Ehud came unto him; and he was sitting by himself alone in the cool upper room. And Ehud said, I have a message from God unto thee. And he arose out of his seat." — Judges 3:20 (ASV)

Probably Ehud’s first message (Judges 3:19) had been delivered to the attendants, and by them carried to the king. Now Ehud is admitted to the king’s presence, into the cool upper chamber.

I have a message from God to you. Ehud believed himself to be accomplishing the divine mandate, and so his words were true in a certain sense. But it was also a stratagem to cause the king to rise, that the thrust might be sure. (The king rose at once, in true Eastern respect for a divine message, or from fear, compare to Joshua 9:24.)