"I was out fishing by the river when I heard music," he begins, peeling off his vest and hanging it by the fire to dry. "It was coming from the middle of the river, where that big willow grows. I was curious, so I got my boat closer."
"Change first, you can continue after you're dry," you tell him, beginning to warm up the vegetable soup and slice the bread.
He heads upstairs to change and comes down a few minutes later with Eloise. She clambers into a chair, thanking you when you give her a bowl of soup and a generous slice of bread. She's a real trooper and doesn't complain about the lack of her favorite fried fish.
"I thought they were things of legend, but there it sat," your brother picks up the story from where he left off. "A Näcken. Would've thought it was just some man off his rocker but he had this air to him. And he was naked as a baby and pale as a river stone. Hard to unsee that."
"Ahem." You raise your eyebrows and gesture to his daughter, who is so engrossed in the story that her bread is going soggy in the soup.
"Did it have big teeth, Papa?" She asks. "Did it try to eat you?"
"I wouldn't know, Pumpkin. I got pulled in the water 'fore I could get a good look," he stares at his bowl, frowning. "I'm a good swimmer, but there I was, about to drown in just a couple feet of water."
"Then what?" Eloise pipes up. "Did you scare it away?" 
"Shouted at him to stop playing the bloody thing but he just kept at it. So I grabbed a rock and threw it as hard as I could. Bam! I got him right across the face." He smacks his hand against the table and the dishes rattle.
Eloise laughs in delight and looks adoringly at her father, the "monster slayer" but you're far from impressed. 
"So you thought taking his fiddle would fix it, did you?" Your voice is cool as your eyes narrow.
"Well, it means he can't play," he shrugs. "Serves him right for trying to drown a good man, I say. Let's see him try his tricks now."
For Eloise's sake, you drop the matter, but your gaze keeps turning to the fiddle leaning against the wall, dripping water to the floor. Once dinner is over and Eloise is tucked into bed, you confront him.
"Brother, that creature is going to be very unhappy that you took something of his. What do you think will happen the next time you go out to the river?" 
"Let him try. I'm taking my hunting knife with me tomorrow," he says, his huge body lit by the dancing flames from the fireplace.
Your brother is a sturdy man and can take care of himself. But the look in his eyes... It's been a long time since you've seen him look like this.
"But—"
"Relax, sister. I'll give the damn thing back eventually," he rumbles. "You know, I... I thought I wasn't going to make it back today. All I could think of was you and Eloise ending up on your own."
You squeeze his shoulder and settle down to knit for a few minutes before it's time to open up the tavern. You're making a new coat for Eloise to wear on cold days. As you knit, you keep glancing at the fiddle, its gleaming surface reflecting the flames from the fireplace. You have some doubts, but you trust your brother when he says he'll give the instrument back, so you let it be.
A few days go by, and you were correct in your assumption that the näcken would be displeased. Your brother doesn't catch a single fish and comes home each day, seething. One afternoon, you decide you've had enough.
"I'm sending it back," you announce as you hang the last of the laundry. "I can't contend with your sulking."
Your brother is sitting on the porch, drinking his third tankard of beer.
"I used to be one of the best fishermen, sister. Now look at me. Reduced to a lout who can't catch a fish to save his life," he groans.
"You can't hope to battle against the supernatural and win," you reply. "I say you're lucky he hasn't drowned you yet." 
"I've seen him a few more times. Just sits on the rocks and laughs at me. I ought to—"
"You stay put." You grab the fiddle from the shelf where you had placed it to keep it safe. 
"I can give it myself, you know," your brother says, but he doesn't move from his spot.
"Ha, I doubt you can walk in a straight line, insufferable fool." You ruffle his hair like you're kids again. "I'll be back."
"Be careful, sister," he calls after you. "If he tries to lure him in, hit him with a rock. It worked for me."
You roll your eyes and start for the river. It doesn't take long to reach, even on foot. The water is framed by grassy banks and spotted with lily pads that float on the surface, brightening the scene with their pink flowers. You can see the willow growing on the small island in the center of the river, but there's no water spirit in sight. You stand at the bank of the river and look around.
"I brought your fiddle back," you call out. "My brother was wrong to take it."
The quiet hum of nature answers you. Birds chirping in the trees and the soft sound of running water. You lift your skirt and wade as far into the water as you dare, clutching the fiddle tightly. The water comes up to your shins and you pause.
"I'll just leave it here for you. Please let my brother fish in your river. My family needs the extra income."
You wait another beat, but you don't see or hear anything out of the ordinary, so you place the fiddle in the water and give it a little push, so it floats away from you. As you turn to trudge out of the river, something rises out of the water right next to you. Your shoe slides on the slimy rocks and you tumble into the water with a little scream. For a moment you don't know up from down—the water is suddenly so deep—but then you're pulled up by a firm grasp on your elbow.
"Thank you," you gasp at how cold the water is, your teeth already beginning to chatter.
The Näcken stands before you, tall and thin, with stringy green hair so long and plentiful that it covers his nakedness. His eyes are the color of bracken water, a murky blue. You can see your surprised face reflected in them.
"Thank you," he says simply. "For bringing back my fiddle."
"It's no trouble. My brother should have never taken it," you reply a little breathlessly.
After all, it's not every day you speak with a supernatural being. He backs away from you and you hastily reach for his forearm. His skin is slippery-smooth and your hand just slides off. He looks at his arm and then at you, his hair falling in his face.
"Can my brother fish again?" You ask timidly.
"I'll think about it," the Näcken replies, tucking his hair behind one pointed ear.
"Well, all he did was take your fiddle, and I have returned it in one piece," you say slowly.
"He threw rocks at me," the Näcken says. "I do not abide any harm attempted on my face."
"I'll tell him to apologize," you say desperately. "Please?" 
"Hmmm. You should go now," he says, sinking halfway into the water. "I am about to play." 
You know not to press your luck, so you turn to leave. You're already out of the water when the Näcken calls out to you.
"Come back tomorrow, and I will give you my answer," he says.
You turn to look at him, but he's disappeared. You can hear the beginnings of a tune, and it's an alluring sound. But the effect is dampened because he keeps stopping to twang at the strings and tune them. You leave before he can begin to properly play.