Let's come up with an amphibian race!

She could also have a skill to drown units, since demons have a mark that punishes.

History.
1st year PC (after the cataclysm) The surviving inhabitants of Tatalia descend into the Dungeon and find exits to the underground sea.
2-10th years. Intensive exploration of the Sea begins.
11th year. The first Sea Kingdom is founded. Ragar I becomes king.
38th year. The death of Ragar I. Civil war. The unified kingdom splits.