Use a worm to gain intel on the army and find two captains that are Bloodbrothers
(located in the 2nd tab). Now, go and dominate
the higher level captain in order to make him a follower. You can also train this follower to make him a higher level and/or shame the enemy captain to bring his level down. Next, you'll want to Command
your captain with a Kill
order on his Bloodbrother. This will generate a mission that you'll want to visit. Additionally, make sure that neither of the captains are on a mission while you assign it, you can Advance Time
if necessary. During this mission, you want your follower to kill his bloodbrother making sure he gets the final blow. You can help weaken the enemy but it is not recommended because this might cause your follower a Betrayal
. A captain might flee, in which case you will need to repeat the Kill order. As your follower kills his Bloodbrother the achievement will unlock.