The Mer (meə(r))

The Mer is refers to the depths of the sea floor, thousands of feet below the surface and miles from the nearest shoreline. It is one of four major regions of A Brief Introduction to Irion, categorized by their differing relationships with magic.
Visiting the region presents a unique difficulty for denizens of other regions. At first, one would expect the threat of drowning and the crushing pressure to be simple to circumvent, as magic that allows one to breathe water as if it were air is fairly simple to accomplish, but a naive explorer relying on their mastery of that spell alone might discover their mistake too late: this is a realm of water and earth, the two elements that contain no inherent magical energy for magi to draw upon, and without that, casting any spell is impossible without access to an alternative source. The first casting they used to bring them safely down from the surface will hold as long as it normally does, but if they allow it to expire before they return, they will find rectifying their mistake far more difficult than expected.
Much of the Mer is a land of darkness where only a handful of lights twinkle invitingly, bright enough to be seen, but dim enough to conceal the terrible hunters who use these lures. Sea serpents, dragon turtles and worse things hunt down here, and woe betide the fool who awakens a mighty kraken from its slumber. Indeed, where those in coastal waters wisely fear sharks, down here a shark attack could be considered a blessing, for it means there's nothing worse lurking just beyond their limited field of view.

Sapient Life

But that is not all there is to this realm. If one knows where to look, one can find pockets of civilization where the peoples of the Mer thrive.
To a resident of the Sleeping Lands, the most recognizable of these are the secretive Sea Elves. Sea Elves are an offshoot of the elves of the surface, who generations ago used magic to permanently enhance their bodies, granting them fins that allow them to glide effortlessly through the water and gills capable of sustaining them as easily as their lungs do on the surface. Their emissaries are occasionally seen in the Sleeping Lands, bearing with them fine artworks and fish that are rarely found near the surface to trade for specially designed reservoirs of magical energy, each capable of granting a magus the capacity to work their spells for a time.
The other Merfolk that surface dwellers are likely to see are the mysterious and dangerous Sahuagin: vaguely humanoid fish-people who occasionally send raiding parties into coastal waters in search for mana cores to craft into their own form of reservioir. These creatures have shown little interest in communicating with the people of the surface, fleeing back into the ocean depths when approached and violently defending themselves if provoked or cornered. No explorer of the Mer has ventured into Sahuagin territories has returned alive, though their bodies are occasionally found near the surface tied to seaweed floats. Trade ships avoid sailing over areas that have been identified as Sahuagin territories, as well as avoiding areas where sightings of their raiding parties are common, though there have been only a handful of recorded cases of ships actually being attacked by Sahuagin.
If those foolish enough to venture into the Mer regularly are to be believed, there are a number of other peoples that make this region their home, though these descriptions are so alien as to defy belief.