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 retrieve Super Mutant Blood Samples for him. These samples can be found on any super mutant you kill, and can be exchanged with Paladin Tristan for caps.

Reward

 * 50 caps and 10 XP per blood sample