Fire Emblem Fates is split into three story paths. Revelation is the third story of the Fire Emblem Fates games and is available as DLC in either of the base games. You need to be have completed Chapter 6 in the either game to access the DLC's path. Revelations has the scouting missions that are available for experience and money like in Birthright but the mission objectives are more complicated like Conquest. In Revelations, the player chooses to not align with either kingdom and it reveals extra information about the Birthright and Conquest storylines.
