The Mantis Corruption

In the east are monsters. In the north is the witch. In the south is the capital. To the west is the waste. The land is called Xin. Life has been nothing but cruel to the children soldiers Sumi and Harakin. They were not expected to survive as long as they have. In the north, a child with shadows ventures to the witch's lair. In the south, an organic mechanic is expelled from her guild.
How will these different individuals from a distant land connect to the events in Books One & Two? And what will happen with the monsters?