Step 1: Unlock "Combat Drain" & "Brand" Abilities (Must Complete "Queen of Shore" Main Mission)
Step 2: Use Combat Drain on a Captain. He must have green health, and this may take many attempts.
Step 3: With a branded Captain, use "Advance Time" until a blue "Recruitment" icon appears on the map. The more branded captains, the more likely this is to appear.
Step 4: Help your Captain with his recruitment.
Step 5: After a successful recruitment, kill all branded (blue) enemies on the mini-map.
This method will also unlock the "A New Master" Achievement: