Magician of the Depths

Magician of the Depths summary:

In a world in which, a long time ago, a mage was said to have been brought to destruction by a monster that ate it. Without parents, Kunimi and Chisayo are picked up by the owner of the region and are made to work. One day Chisayo receives a harsh punishment. Kunimi who thought that it won’t be a good idea if this continues, takes his sister and runs away. But then, they are swallowed by a large sudden crack in the ground ?