Each level has 3 modes with escalating difficulty: the Iron Skull, Emperor’s Laurels and Crux Terminatus. To unlock the Emperor’s Laurels and Crux Terminatus modes on each level you simply have to finish that level. These Modes are different ways to play a level and can be things like Meteor Showers, Increased Enemy Health or Restrictions on Weapon Usage which completely change the level experience. Do well in these Game Modes and earn additional Stars. There are Fireteam Missions for each mode as well; to qualify for the difficulty-specific Fireteam Missions you’ll have to finish the mode you wish to attempt with 3 Stars.