Abner was the son of Ner and uncle of Saul. He became the commander-in-chief of Saul's army when Saul became king. It was Abner who first introduced David to the court of Saul after the victory over Goliath.
After Saul died, David was made king over Judah. There was a lot of hostility from the other tribes and Abner, at the head of Ephraim, helped foster it. It was his desire that Ish-bosheth, the youngest son of Saul, be king and he succeeded in making him ruler over Israel.
Then one day, soldiers of the two rival armies - one led by Abner, the other led by Joab - met across the pool of Gibeon and decided to fight it out in hand to hand combat. Joab's men were victorious, and Abner fled, pursued by Asahel, Joab's brother.
He chased Abner, turning neither to the right nor to the left as he pursued him. 20 Abner looked behind him and asked, "Is that you, Asahel?"
"It is," he answered.
Then Abner said to him, "Turn aside to the right or to the left; take on one of the young men and strip him of his weapons."
But Asahel refused to give up the pursuit; so Abner thrust the butt of his spear into Asahel's stomach, and the spear came out through his back. He fell there and died on the spot. And every man stopped when he came to the place where Asahel had fallen and died.
(2 Samuel 19-21,23)
A ceasefire was declared immediately after and the two armies returned to their homes.
A while later, Abner was rebuked by Ish-bosheth for the impropriety of taking Rizpah, a concubine of King Saul, as his wife. This made Abner cross sides, going over to David who received him favorably. It was a mistake, because Joab killed him in revenge for the slaying of his brother.
David mourned his death in public.
The king sang this lament for Abner:
"Should Abner have died as the lawless die?
Your hands were not bound,
your feet were not fettered.
You fell as one falls before wicked men."
And all the people wept over him again.
(2 Sam. 3:33-34)