Mercenary Event Summons include a Bonus Promotion Item Chest that is awarded for every 10 summons. You can claim a maximum of 9 chests during each Summon event.

The Bonus Promotion Item Chest includes a guaranteed Rare or Epic Promotion Item. The chest can also contain Rare or Epic Crafting materials.

Your progress towards the next chest will carry over to the next Mercenary Summon event if you still have chests in stock. Your progress is shown in the Mercenary Event Summon view (look for a chest icon). Tapping the chest will show you the amount of chests in stock.