blood oath

:  an oath taken by two or more people in which they ceremonially use or exchange each other's blood; broadly :  a very serious or solemn oath … she'd have to sign a blood oath promising that no matter what happened she'd never leave once that child was born … — Ron Harris, Elle, March 1993 Back when we were 17, we all took a blood oath to record three albums of 30-minute punk rock, and nobody would get to leave until we'd done just that. — Pelle Almqvist, Spin, November 2007