User blog comment:PixelKnight4866/Fire Emblem Heroes/@comment-5107974-20180307142240/@comment-4199482-20180308065704

They just added a new mode called Rival Domains with larger battles. They mentioned using 20 of your heroes per battle. It was vague, but either the maps are bigger for that mode or it's like "Enemy Reinforcements" but for the player. I'm gonna try it out right now to see. Maybe it's something you'd like?