blood oath

An oath of loyalty between two or more people, especially one made in a ceremony in which small cuts made on each person's hand, finger, or arm are pressed together so that their blood can comingle. Tom and I formed a blood oath swearing that we would both keep the secret of the hidden treasure between us until the day we died. Let's all make a blood oath that we'll remain friends forever, no matter what! I made a blood oath with the king in my youth and thus will forever defend him to the utmost!