Brand 5 Bodyguards of a Warchief, turning them against him in combat.

First you need to unlock the Brand ability from playing story missions. Once you have the ability find 5 captains, dominate them and command them to "initiate" the same Warchief. You will have to complete an initiation mission for each captain which will then promote them to bodyguard. When a single Warchief has 5 bodyguards find the Warchief's mission. Once you see him with his bodyguards strike the Warchief and the achievement will pop.