Joab (יוֹאָב "The LORD is father", Standard Hebrew Yoʾav, Tiberian Hebrew Yôʾāḇ) was the nephew of King David, the son of Zeruiah in the Bible. He was made the captain of David's army (2 Samuel 8:16; 20:23; 1 Chronicles 11:6; 18:15; 27:34). He had two brothers, Abishai and Asahel. Asahel was killed by Abner, for which Joab took revenge by treacherously murdering Abner (2 Samuel 2:13-32; 3:27). Wikipedia