Fire Emblem Heroes is about to have its first big event in the form of a Grand Hero battle, announced via an in-game notification. Not much is known about the event at this point, however.
In any case, the battle is scheduled to begin on 10 February 2017, running until 14 February 2017. Although the opponent is hidden at this point, it appears to that person in question is Narcian, a Wyvern General who first made an appearance in Fire Emblem: The Binding Blade.
From the sound of it, the event is going to the first actual challenge, so be sure to have a lineup of heroes suited for the job, and by which we mean having a team of Hectors and Takumis. Fire Emblem Heroes currently available for iOS and Android.