Make a Captain kill his bloodbrother. To do this, find a captain who has Bloodbrother under his hate Traits and make him your follower. Next, command your new follower to kill his bloodbrother. This should start a mission or a pit fight. If it start's a mission you can't interfere with the fight as your follower will turn back and you will miss your trophy. Just sit back and watch and your Trophy will pop once your follower has killed his bloodbrother.
Here is a video guide for help if your having any troubles.