After a battle against shadowy enemies, you will be asked to make a decision.
Choose a Path
If you bought Fire Emblem Fates: Birthright, then you can play "Defend Hoshido" for free. If you bought Fire Emblem Fates: Conquest, then you can play "Fight with Nohr" for free. If you want to choose the other (non-free) option, you will be prompted to purchase the other path, and you will get a discounted price by purchasing it from within the game this way. You can also choose "Refuse to choose a side" and you will be prompted to purchase the DLC-only Fire Emblem Fates: Revelation.
The next part of the walkthrough depends on which path you chose: