Verti, the Siren & the Crystal Cave

This is a story we tell our children the moment they can walk & listen; the moment they can feel the cold on their skin and can depict the bite of the frost; the moment they feel our love and the harsh cold light of the sun.
This is a story we tell ourselves to remind us of the dangers of the cold dark waters and the even darker caves beneath us. - Sit, Stranger, hear about Verti, the Cave and the Song.

— Old Frost Maiden to a traveller in the Northern Realms

Verti was a strange northman. Sure, he dressed like us, fought like us, fu...slept like us and by the Frost Queen and her husband the Fire King, he could drink like none of us. Even with the bow he was far better than anyone of us.
But Verti was a strange soul. A kind one, if there is one under the Northmen. He was built like a brickhouse of the weaker ones in the south and could knock out a wild bear with one of his fists.
He was a strange one. He bought paper and ink and quills from merchants. His hutt was a mixture of deadly weapons and a large wall where one could think there lives a scrive. He also categorised and wrote everything down he saw, starting by rocks over forests, landslides and animals.
One day he took a liking in the caves north of our village. We don't go there because of slippery rocks and the tide. It could drown you or pull you out in the open sea where you will likely drown as well. Or freeze to death, if you can swim long enough.
Anyway... Verti found a hole in the ground and some natural stairs. Rocks in a specific pattern. Sometimes he used them, sometimes he used the hole. Both led into a cave, illuminated by the sun. Not large, it was just the entrance to a large cave system. There he ventured once every week, bringing back small crystals, which emitted a faint blue light.
Hm, yes, the ones you see in our lamps, that is right. - Where was I? Oh yeah, right. You see, merchants are a rare sight in these lands. Puny little southerners, can't withstand a little bit of ice and wind. Pah.
Well, Verti found one of this merchants and showed them the crystals. The merchant was not interested, but his wife... she was a good one if there is a southern woman who is good, you see. She saw the value of these crystals, bought them and returned a year later, bought the entire stack of crystals Verti brought out of the cave.
Over the years Verti emptied cave by cave. He had to be careful, the deeper he had to go the earlier he had to return before the tide would flood the caves again.
One day he reported that he had found a cave which he called the main cave because it was so vast, he couldn't see the ceiling or the end of it. We think it lays somewhere under the hills in the northern west. Yes, with the statues, correct.
So, Verti informed us about the cave, told us about the location and security measures he had taken before returning into this labyrinth. He worked his way into the main cave and started in one of the smaller entrances. After an hour or so he had a few crystals and decided, he needs only two more before he heard a female voice, as clear and beautiful as the crystals.
She wasn't talking. She was singing. A song of old, untold worlds in the tongue of the Gods. A song of love and life, loss and sorrow. Verti was curious, who would sing in such a harsh environment? And who would venture into the cave and sing?
He followed the voice, his torch let the crystals glitter like the stars in the nightsky. He followed into a smaller cave where the ocean swept in. And there, in a small lake, he saw her, her silhouette caressed by the flames of his torch.
She was beautiful. A naked woman with ice blue skin, eyes so blue they were nearly white. Her hair was flowing over her shoulder like liquid silver. She laid with her upper body on the cold stone, her arms crossed and her head rested on her arms, singing and smiling at Verti.
He asked her why she was there, laying in the icy water and singing. Where does she come from, what was her name?
Come closer and I will tell you my name, she sang, reaching out to his head and pulling him closer, so soft and tender like a mother would embrace her child. She kissed him und pulled. Verti had no chance for she was a Siren and stronger than even the strongest man and Verti was a strong man.
He didn't mind, even as his lungs screamed for air, for the fresh relieve of breathing again. He minded her kiss, her soft tender lips and the sharp, pointy fangs she showed as they were so deep under water light was only a memory.
He didn't mind as four other beautiful Sirens pulled closer, kissing and biting and eating him while he was drowning.
Verti wouldn't mind again.

How do I know all of this? What happened? Oh, sweet traveller, don't you see what I am? I am a Frost Maiden. Of course the ice told me what happened, silly.

— The Frost Maiden telling the story of Verti