The story of Ish-Bosheth's death can be found in 2 Samuel, Chapter 4. Ish-Bosheth was one of the sons of Saul. After Saul was killed in battle, Ish-Bosheth was proclaimed as king by Saul's faction. They fought the tribe of Judah, which had proclaimed David as king. David's side won and he became king of Israel.
After this, two of Ish-Bosheth's men, named Rekab and Baanah, decided to kill him. They entered his house while he was asleep and stabbed him in the stomach and cut off his head. They brought the head to David, presumably expecting a reward. But David had them killed for murdering what he called an innocent man.