Take control of the Stronghold by Dominating the local Warchief.
Make Uruks kill each other quickly to draw out the Warchief. This mission will take place within the confines of a Stronghold. You will need to be quick and nimble and brand many enemies so that you will fulfill the goal of having 50 Uruks killed by branded Uruks. Once you achieve this, the Warchief will come out of hiding.