Fire Emblem Heroes Book Four

From the introduction:

In the beginning, there were two families blessed by dragons. One bore the power to open pathways between worlds, and the other to close them. There was balance. But things changed, as they are wont to do. There was war between these two countries, and that was likely where you came into things. But soon after here was also fire and ice, and there was death itself. And in the days to come there will be beings made from dreams and nightmares doing battle, with Askr and Embla as their battleground.