http://www.pcgamer.com/shadow-of-war-infiltration-missions/

One of my favorite things about Middle-earth: Shadow of War so far are its variety of mission types. I've just hit Act II and completed my first fortress siege, a multi-stage battle where you bring your loyal (read: magi-psychically dominated into loyalty and subservience) orcs into battle against a powerful overlord, who is served by multiple war chief lieutenants.

To prepare for that big battle, I commanded one of my orcs to fight the bodyguard orc of one of the war chiefs. If your orc defeats this bodyguard, Shadow of War gives them the opportunity to become a spy, and ambush that war chief if and when you go to fight them.