You make your way through the Woods of the Forlorn in the hope of quickly reaching Nohr, but you face an enemy.
There are marsh tiles throughout this area, and Faceless will randomly spawn from them. The marsh tiles will also damage your units if they end their turns on them.
In the north half of the map, Leo is flanked by his retainers Niles and Odin. Defeat Niles and Odin to get some dropped items.
Leo can use the Dragon Veins in the north part of the map to create more marshes, which in turn creates more Faceless for you to fight and more hazardous tiles to avoid standing on.
Send someone with high Res to fight Leo. The battle will end as soon as you defeat him.