Wadatsumi is an island found on on the South-east side of the continent of Babel. It is a state found in constant turmoil, as recent history has left it devastated, and the crushed populace is left dealing with an invasion of Demons.
See the exact location of Wadatsumi below:
Wadatsumi is the main featured location of Part 2, Chapter 2, whose protagonist is Amane.
For all the Units from Wadatsumi, see here