When close to the final missions, you will learn how to issue commands to branded units. Brand any existing bodyguards of a Warchief, then brand Captains who do not yet have a Warchief they are guarding. After branding a Captain, dominate it immediately and issue a command to have them try to become a bodyguard for that Warchief. The Captain will then need to complete a trial by ordeal mission to become a bodyguard. Once you have obtained five bodyguards for the same Warchief, enter that Warchief's mission. "Stinking Rebels" will be earned when you get him to appear.