Deep One

Naoya and his twin sister Konoka had lost their parents at a young age and had a tough childhood. Nevertheless, they still managed to find happiness in the little things in life and wished that these warm peaceful days would last forever. However, this faint hope was mercilessly crushed when a Grimoire suddenly appeared before them. It is a forbidden magic book that has existed since the beginning of time, but which should not be in the human realm. They thought that they were an ordinary family, but they were actually the heirs to the Old One, which has the power to destabilize the world.