Armoni is the son of Saul (king). When Saul attacked the people of Gibeon, his sons had to be killed, and Armoni was one of the sons chosen and executed at that time.
2 Sam 21:8So the king took the two sons of Saul by Rizpah the daughter of Aiah, Armoni and Mephibosheth; and the five sons of Adriel the son of Barzillai the Meholathite, whom he had by Saul’s daughter Merab.