Also: Mermen, Merpeople, Merians
One of the two races descended from a common ancestor brought to Earth by the Old God, the Maerum are ocean-dwelling folk ruled by the Maerum Queen. They are a matriarchal society, where the females protect the males. The Maerum April visits live in the Sea of Songs in a magnificent undersea city which they call the "Third City".
At the time April arrived, the Maerum were suffering because the luminescent Tanyenn algae that grew around the city was dying out: the light from the Tanyenn attracted fish for the Maerum to catch and kept large predators like the Snapjaw away, so the loss of the Tanyenn was causing them great difficulties. The Maerum were also at war with the Alatien, with the two races calling each other "Wettails" and "Wingdemons" respectively.
The Maerum have a strange reputation in Marcuria as 'bloodthirsty cannibal mermen' according to the sailor Umber Ianos, but they don't actually eat people (or each other). Instead, they seem to 'rescue' shipwrecked sailors by bringing them down to their city, as April experienced firsthand. Once there, she was put inside the "Landwalker's Bubble" which was full of air, and decorated with diagrams showing her how to use a polyp in the wall to breathe underwater, and how to use the Tanyenn to communicate with the Maerum. The Maerum Queen told April she would eventually become a "Gatherer" in the city. This suggests the Maerum have a class of slaves, possibly composed entirely of shipwrecked humans and Dolmari, giving rise to the legend of salvation by a "Landwalker".
The Maerum worship the Old God, who is actually the gigantic Blue of the Kin who dwells in the deeper oceans. They also venerate the prophecy of the Waterstiller, whose identity will be confirmed by the completion of four tasks: bringing proof of her mission to the Balance, killing a Snapjaw with her own spear, finding the piece of the Stone Disk the Maerum hold, and 'reuniting their people'. The Queen points out that April cannot be the Waterstiller, because the Maerum are at peace amongst themselves, but April finds an abandoned shrine with proof that the Maerum and the Alatien are descended from the Old God, and therefore this 'reunion' means bringing peace to the two races. Once this is accomplished, Tanyenn will become plentiful again and both races will flourish.
Interestingly, at VAVA in Venice, April comments on Emma's 'absolutely beautiful' holosculpture that, for an instant, comes to life when her back is turned. The sculpture closely resembles the Maerum Queen. Also, Fiona tells April she had seen an "underwater city" when looking into the canals of Venice.
The original TLJ site had this introduction to the Maerum.
"Landwalker. The currents of the ocean have brought you here for a purpose. The lights are fading in our city. There is no tanyenn left in our homes. I am afraid we will not be able to hold off the snapjaw much longer with our stingers alone. We wait for the Waterstiller. We dread the coming storm.
"You speak to us through magic, landwalker. Our tongues are made for water, yours for air. But magic is the common tongue. There is no magic where you come from? In this world there is only magic. And the storm of chaos."
"The storm? It comes slower down here, but stronger. Chaos is loose, and the currents speak of distant turmoil. We are afraid, landwalker. Afraid for ourselves and our children. Every day the snapjaw come closer. When we search for food, we come back with one less hunter than we set out with. Even offerings to our ancient god do little good. He sleeps in the deepest depths of the ocean. He is our vessel. Some day we will travel to distant seas on his back."
"Tanyenn is spirit and life, in harmony. Tanyenn is what gives us light and nourishment and protection from death. But there is no tanyenn left. We need your help, landwalker. Only you can bring us the matter we need for tanyenn to grow on our scales. In times past, it was not like this. Tanyenn was plentiful. We had to tolerate the mindless squawks of those flying pests, the Alatien people. But life was good."
"Do you know the Waterstiller, landwalker? The Waterstiller is of your people, or so it is said. Perhaps it is you? Do you carry the sign? No, you do not. You are not the Waterstiller, but you can still help us. Hurry, before the lights go out and the snapjaw attack in the dark of night."