Blood Brotherhood

 refers to an unmarked and repeatable quest in the Fallout 3 DLC Broken Steel.

Walkthrough
After the completion of Who Dares Wins, Paladin Tristan asks you to kill Super Mutants and bring back Super Mutant Blood Samples as proof of death. These samples can be found on any super mutant you kill, and can be exchanged with Paladin Tristan in the Citadel's A-Ring for caps and XP.

Reward

 * 50 caps and 10 XP per blood sample