What military leader was killed while holding on to the horn of the altar?

The military leader who was killed while holding onto the horn of the altar was Joab, the commander of King David's army. According to the biblical account in 1 Kings 2:28-34, Joab sought refuge at the altar to escape execution ordered by King Solomon. Despite his plea for mercy, Solomon sent Benaiah to carry out the execution, resulting in Joab's death at the altar.