In Klingon culture, a blood oath was 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. According to the oath, the four swore to cut out the heart of the Albino and eat it before his still-living eyes.
While Koloth later admitted to Jadzia Dax in 2370 that "Curzon Dax was a politician" and that "It was a diplomatic ploy for him to bond himself to us in a blood oath," however Jadzia assured him that assessment was untrue. A short time later, their blood oath was fulfilled when they tracked down the Albino to Secarus IV, and successfully killed their enemy. (DS9: "Blood Oath")