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 for caps and XP.

Note that the dead Super Mutant corpses littering the Statesman Hotel during the quest Reilly's Rangers will contain blood samples for the most part. This has yet to be confirmed to work if you've visited the place prior to obtaining this quest.

This is one of the most effective ways to gain exp and caps. A mere 40 samples is worth 2000 caps! (this can be easily obtained in the Capitol Building)

Reward

 * 50 caps and 10 XP per blood sample