After Abner died, Israel was thrown into a bit of chaos. Two of Ish-bosheth’s captains, Baanah and Rechab stabbed and beheaded him one day while he was taking his afternoon nap. They took the head to David in Hebron, expecting to be rewarded. But David was all, you’ll get the same reward as the guy who told me he killed Saul. Guards, kill them!
Then David had the head buried in Abner’s tomb.
By the way, there was still one other relative of Saul’s still alive. A son of Jonathan named Mephibosheth whose nurse had fled with him when his father was killed. Unfortunately, she dropped him (he was only five) and caused him to become lame in his feet.