When you complete a stage you will get the chance to try your luck to get one of three reward items. This reward may contain Hero's equipment or a unit. You can also spend 10 gems to draw your cards again. In order to get the reward after completing a stage, claim it by tapping the item icon after selecting the stage. After doing so, the "clear" sign will appear. Then, go to your mail and open the chest that was sent to you. You must repeat the process each time you clear a stage again. The Deadly Boss randomly appears when you clear a stage. Defeat it to get the Chest of Empire. You can get six-star equipment and elite units from a Chest of Empire.