During a Mercenary Event, in your Summon Gate will be the chance to Summon a Special Event Heroes. The Event Summons are timed, just like the Mercenary Events, but otherwise work just like your other Epic Hero summons. 

Please note that all heroes included in Epic Hero Summon are in the Event Summon roster as well. Each Event Summon has also a chance for a free extra summon of the Hero of the Month.

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

Although the chance for one of these Heroes is rare, the good news is if you do receive one, it is yours to keep, even after the Event is over.