Note: The entire book is free. The earlier chapters may be hard to stomach, and the later chapters are highly disjointed, so read with caution.
Before the black tide, a girl stands with her sword raised. She gazes at the despairing fleet in the deep sea, then lowers her eyes to study her reflection on the water. She once looked human. Driven by regret and bloody vengeance, she became part of the deep sea for a time. When she awoke once more, the world had cut all ties with her.
A long dream ends, and abyssal beasts roar across the world. The meaning of responsibility has passed down through the cycles of life, generation after generation, until it reaches this point.
(I just made up the blurb.)