The Lord of the Plains by Sarah Chapman
Summary: In an empty and dangerous world, the human city of Astar shines as a lone beacon of peace, free from the constant warring of the inhuman, gemeng tribes of the Plains. Aerlid, a man neither human nor gemeng, tricks his way into Astar with his charge, a young gemeng named Riley. In the human city, she experiences a type of fear and oppression foreign to her, and longs for the freedom of the world beyond the city walls.
Unbeknownst to her, the world is changing. To the north, another human city is discovered, where the people do not constantly fear invasion by the gemengs, while in the western city of Coastside, a young man begins speaking to the fishpeople, gemengs that dwell within the ocean.
But something far worse is coming. The ancestors of the gemengs, creatures far more powerful than gemeng or human has faced in centuries, are returning. Unaware of the danger, Riley abandons the safety of Astar, while the people of the city remain sure in the strength of their walls…