Blood Brotherhood

 refers to an unmarked and repeatable quest in the Fallout 3 add-on Broken Steel.

Walkthrough
After the completion of Who Dares Wins, if you did not destroy the Citadel, Paladin Tristan asks you to kill super mutants and bring back super mutant blood samples so the Brotherhood could research a weapon that could destroy super mutants with ease. These samples can be found on any super mutant you kill, much like ears and fingers on corpses, 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.

Bugs

 * Occasionally, Tristan's dialogue for exchanging the blood samples for caps will go normally, but neither blood samples nor caps will be transferred. This can be solved by not speeding through Tristan's dialogue during the actual transaction event.

Behind the scenes
The name, Blood Brotherhood, is of course a reference to the play Blood Brothers.