Nenlindo, Mermaids. Are a people of Ulmo. A people known for their ethereal beauty of voice and appearance. They were created during the early days, but they were already splintered when Ossë lead a few of them into the arms of Melkor. Those became sirens, leaving their beauty, the sea and water for wings, the cliffs and skies, which they rule with their beautiful voices.
Since Mermaids lived in the sea that was the limit for others, they could roam freely. While many remained in the open and safe sea, some travelled inward, to both Middle-earth and Aman. The fresh-water mermaids of middle-earth were darker in colours, featuring fish-like patterns on their body with murky and natural colours that would hide them in the rivers and lakes from potential human poachers. Their bodies are broader in shoulders and they are very powerful creatures, with the ability to throw a spear over many miles.
The salt-water mermaids, living close to the coast of middle-earth, who had never seen the light of Aman, were the most colourful of them all, with brilliant colours of the entire rainbow, from oranges to pink and turquoise. Whilst the ones living by Amen’s coastline, Dûnnenlindo, were less colourful, pure light with faint pastel colours. It is the same for the fresh-water amen mermaids.
Unlike their middle-earth counterpart, these can use various abilities, such as telepathy, water manipulation, partial-shapeshifting (from fishtail to legs), aquatic perception (the ability to sense everything that happens within the body of water you are focused on) and partial precognition. Their alcohol tolerance is also impossibly high, since they, like the elves, are thought as a more sophisticated group. Many mermaids who walk on land have the bonus of being able to walk on every shape of water without even making a dent on its surface.
The body of a salt-water mermaid was also built differently, more streamline to accommodate for the bigger distances they could swim, which meant they are less broad in the shoulders and are often thereby likened to elves with their petite physique, yet it has no influence on their strength.
But there are many weaknesses that come with this: their heat tolerance is lower than that of other races, salt is also something they do not tolerate as much (mostly only the fresh-water ones) as it can lead to visions and temporary paralysation. The new moon is their weakest point of the month, but the strongest for a siren.
History hasn’t either been pleasant to them, mermaids had to fear both sirens and humans for a long time. Since they would often be hunted down, slaughtered and used for entertainment. But there were also many wars between humans and mermaids over the ages, the most current one was almost 700 years ago, the battle by Edhellond. That was the last time a mermaid has been spotted and they faded into memory, becoming a myth, dangerous and vain monsters of the sea.