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, 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, 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 or Glitches
When you bring some back (20 or more), Tristan dialogue for exchanging the Blood Samples for Caps will go normally, but occasionally, neither your Blood samples nor the Caps will be transferred. Solution: Dont speed through Tristan's chat dialogues during the actual Transaction event.
 * After completing the Broken Steel DLC and Tristan asks you to bring back Mutant Blood Samples