In Klingon culture, a blood oath is a vow of vengeance that must never be broken. Kang, Kor, Koloth, and Curzon Dax swore a blood oath together to take revenge on the Albino for killing the Klingons' firstborn sons in 2290. Their blood oath was finally fulfilled in 2370, when they tracked down the Albino to Secarus IV, and successfully killed their enemy.
According to the oath, the four swore to cut out the heart of the Albino and eat it before his still-living eyes. (DS9: "Blood Oath")