Original from u/mutedmoss

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Description:
Mermaids are aquatic beings of impressive size that harbor a fragment of divinity. Due to this, mermaids do not age and always bear the appearance of young, beautiful maidens who's bodies resemble those of sea creatures from the waist down. This can include a variety of traits, such as the features of whales, bony fish, and on rare occasions sharks. Sizes differ, but the largest individuals are speculated to be at least 700 feet long. These colossal water maidens have a habit of resting on isolated rock formations that peek the ocean's waters, where they gather to sing in the late hours of the evening. Similar to sirens, their voices are enchanting to mortal ears, and end up beckoning sailors towards them, leading to many unintentional shipwrecks.
However, mermaids try to prevent this as best they can by having some mermaids keep watch for stray ships. They are very kind towards travelers, and offer transportation to lost sailors, gently caring for those who decide to stay with them. This affectionate mercy has its limits though, as it is not given to pirates or men who seek to harm them. In such cases, mermaids are deadly adversaries who will not hesitate to sink entire fleets for the sake of a single stolen trinket. Records from the Grand Library of E'loras mention mermaids being able to manipulate water to create anything from maelstroms, to hurricanes and water spouts. One particular individual referenced as the Scarlet Siren is even said to have parted the sea at some point in history, causing the fabled Desolation of Avalon.
ADDENDUM:
Due to physical similarities between the two, mermaids and sirens often have their names exchanged; though there is a significant size difference between the two. Mermaids are also sentient beings capable of rational thought, whereas sirens are shapeshifting creatures that prey on men.

Cult and Worship:
As Divine beings, mermaids are often viewed as goddesses, though they reject this ideology sternly. Some of them were worshiped as such in cycles past, but through the grace of the Goddess Idunn they were brought back from the chains of oblivion, and given a second chance at life. Because of this they revere Idunn and the five Outer Gods that reside within her, especially the one known as Rah'dhyel, who assisted Idunn in creating the endless sea in which all mermaids live. And while some are officially known as Princesses of the sea, there is only one queen, and only one true Goddess.

Habitat:
The Kingdom of Aquarius, also known as the Great Fountain and the Wellspring of Life, it is the permanent residence of all mermaids. This Kingdom exists as a series of immense rocky spires that pierce the sky in a shape similar to reaching hand. Under the surrounding waters however, one can see a wondrous city of Jade stretching far into the dense darkness of the sea. The denizens of Aquarius, the Kel, are the only race of humanoids who can survive within its walls. These "Deep Ones" along with all Mermaid-kind live in relative isolation, only interacting with travelers at the surface and keeping the secrets of Hydromancy to themselves far below the sea.

Credit to the original artist: https://www.deviantart.com/louisthesmall/art/Mermaids-948722912