During a Challenge Event, in your Summon gate will be the chance to Summon a special event hero. The Event Summons are timed, just like the Challenge Events, but otherwise, work just like your other Epic Hero summons.
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.
All bosses faced in a Challenge Event are also available in the Event Summon. The top Event rewards also include some of these Heroes.