Directly to you from Twitter:
Another Grand Hero Battle has arrived! This time you'll take down Ursula. Win the battle on Lunatic and receive a 4… twitter.com/i/web/status/8…—
Nintendo of America (@NintendoAmerica) March 10, 2017
…Wanna play? Buy a 3DS.
That being said, click on Fire Emblem Heroes for everything you need to know about the game. 😀