The Crushing Deep

Far south of Diamus, deep into South Herra's Grave, lays a waste of terrible entities that harbor this place. A large kingdom that Susanawoo obliterated with his arcane abilities, every being was killed here and was shifted into something new. This was his first stop in his terrible onslaught of the innocent underwater dwelling people, and quite a many of their items stays here, waiting to either has more added to it or to be discovered by someone strong enough.
Underwater demonic entities took over this area and changed the Merfolk that lived there into scary beings. What once used to be a thriving series of towns with happy mermaids and mermen, now has ghastly and gnarled creatures that could strip the meat from the bone in moments; and those are least scary things in there. The most stealthy sea divers have ventured here and more have not returned. Those that have talk of the scariest seafaring creatures they’ve ever seen but also the rarest of underwater treasures. Artifacts made by Merfolk beings long since dead, gold that’s so exceptionally rare they’ve inflated 3 times over into current times. The few that have come back alive gained riches and stories that others would be jealous of- or, in some places, would want to ruin and steal.

Geography

Deep-sea and broken castles. Two currents go in but only one large current goes out, and none of the ghastly creatures seem to be able to leave.