After meeting the Rainbow Sage, you try to reach Nohr through Fort Dragonfall.
Defeat the Boss
In this map, there are acid-bog tiles that will harm your units if they are standing on them at the end of the player phase. In addition, Faceless will spawn out of these bog tiles.
Throughout this map, there are Dragon Veins that your royals can use to remove the acid-bog tiles.
Some of the Faceless in this map will drop items when you defeat them, so be sure to check what they are carrying.
There are also some chests in this map. The Faceless can open these chests, so be sure to hurry over and use your Locktouch units to open the chests before the Faceless can. Otherwise, be sure to kill the Faceless that steal anything from the chests.
The boss unit can avoid damage from most weapons. Beaststones, Dragonstones, Shurikens and Daggers are the only effective weapons against it, so use those to get rid of the boss unit. The battle will end as soon as you defeat it, so make sure to get the treasures from this map before you do so.