The Matyll Siren
The Matyll Siren supposedly helps advernturers in. She is said to have been banished from the Feywild.
According to the myth, inside the Somnium Lake lives a Siren that helps out lost explorers by singing them a song that leads them out of Matyll Valley. Only with your last glance you might be able to see her true form.
According to multiple adventurers, a Siren appeared to them when they had already given up hope. They were walking along the Somium Lake, and upon hearing the Sirens call didn't even care about the consequences. But contrary to their belief, they didn't get drawn to a beautiful woman, but rather outside the valley. They all claim to have seen the shape of a female figure when they last turned around to watch the lake.
The myth was spread by the saved adventurers, and then continued to spread through taverns and the like.
Variations & Mutation
On the darker side, sometimes the myth gets told as a version where the Siren leads explorers out ofin exchange for their soul. Also, her shape varies in every tale. According to some, she is an incredibly beautiful woman, while others describe her as the ugliest hag that ever existed.
Most people tend to shrug this off as "just a myth", but the people that have met the explorers that got saved are sure that the myth is true.