Antiomno was a Lamanite king who ruled the land of Middoni, around 90 BC, and was a friend of King Lamoni of Ishmael (Alma 20:4). The Nephite missionary Aaron, along with Muloki and Ammah, had been imprisoned in Middoni while preaching there (Alma 20:2). When the voice of the Lord directed Ammon to go to Middoni to deliver his brethren, Lamoni offered to go with him, saying that because Antiomno was his friend he could get the prisoners released (Alma 20:4).
After traveling to Middoni, Lamoni found favor with Antiomno, and Aaron and his companions were brought out of prison (Alma 20:28-30).