‘No-one really knows when it started or why,’ says Ruth Rieder, showing me round the museum devoted to Tschäggättä she runs with her family. ‘One story is that on the other side of the valley, it is shadowy and nothing grows, so people would come over here to the sunny side in disguise and steal the food.’

As the cold winter air creeps through the stone walls of the building, she leads me through rooms devoted to the different parts of the Tschäggä’s costume: shoulder braces once used to carry wood that give the beasts their height; shawls made from the hides of goat, sheep, cow, even St Bernard; embroidered leather belts with heavy iron cowbells hanging off them; long trousers made from sackcloth. And then there are the masks, 400 of them – piled in corners, looming out of walls, staring up from work benches, each bearing the idiosyncratic marks of their creator, all equally and uniquely unsettling.

The oldest the Rieders have found dates from 1892, though the tradition is believed to have started long before then. ‘Nothing older is left,’ says Ruth. ‘People were poor here, and it was cold, so they burned them afterwards.’ The Rieder family has been more responsible than most for keeping Tschäggättä alive in the valley. Her mother-in-law was the first female mask-carver in the valley, and her husband Heinrich is now the most celebrated, famed for his fantastical and macabre creations.

He has already finished his mask, made for a participant in the climax of this year’s festival in a few days’ time - the daytime parade of monsters, in which prizes are awarded for the best. Ruth gives nothing away about their plans. ‘No-one should know who the Tschäggä is,’ she says as we leave the museum. ‘Only the families will know what the costumes are and who’s wearing them. You will only know that if you see one, you will be afraid.’

A fat slug of cloud sits over the village of Kippel. A postwoman makes her deliveries on foot, navigating a path hacked through the snow. She stops to chat to a dog-walker carrying a basket of logs, the Jack Russell at his feet impatient to get moving. I enter a rambling cluster of outhouses and barns, the air sweet with the smell of manure. Joining the burbling of chickens and bleating of sheep is another sound: the tapping of chisel on wood.

Albert Ebener is busy in his workshop. Leaning over a lump of local Arvenholz (Swiss pine) held firm in a vice, he deftly cuts away at a row of teeth emerging from a face, wood chips flying into the air around him. With his gnarled face, he looks carved from a piece of wood himself.

Albert has been working with wood for most of his seventy years. ‘My grandpa carved masks,’ he says, puffing on a pipe that rarely leaves his mouth. ‘In this valley, if the grandfather carves, the father carves, and the son will also carve.’

Among boxes of screws and deer antlers, and racks of drills and planers are masks made by his father, uncle and grandfather – hideous things with mouths filled with wonky cows teeth, permanently howling beneath moustaches of rabbit fur. I try one on and am surprised by how heavy it is, my head pushed into my neck under its weight. My field of vision shrinks to the two tiny spots of the eyeholes. The masks are made smaller and lighter now, and are influenced as much by Hollywood horrors as Swiss folklore and the individual imagination. ‘They are still horrible though,’ says Albert with a laugh.

He reckons to have made six thousand masks for Tschäggättä, and to have joined countless parades – though those days are now behind him. With the costumes weighing 7kg, this is a young-person’s game. Traditionally, the monsters were unmarried men, allowed out only in the daytime and never on Sundays, but Albert was part of a movement to change that. In the 1970s, he and other men set out after dark to protest the strict rules of the commune.

The ban on married men and women and on night-time prowlings was overturned (though Sundays remain out of bounds, in deference to the church). The victory is marked every year on the final Thursday of the festival, with a nightly procession from the last village in the valley to the first. ‘It was special to be part of it,’ he says wiping sawdust from the unfinished mask. ‘But not much changes with the tradition really. We still do it because we love this valley and are proud of it.’

On the day of the procession, excitement in the Lötschental is building. Gangs of skiers and snowboarders wait as usual at the gondola in Wiler, ready to be whisked off up the mountain to runs far out of sight. But among them roam tiny, child Tschäggäs – two-foot-tall monsters who run growling up to pedestrians and block the progress of cars, flying into a rage when their victims find them cute rather than hideous . Every restaurant and tavern has some sort of carnival paraphernalia on show, and stalls selling hot wine and soup set up on the road. In an old bull pen in Wiler, Manuel Blötzer and five friends are getting ready.

They asked the commune if they could take over the old building and turn it into their Tschäggättä ‘cave’, a place where they could store costumes, discuss ideas and get dressed. One by one the boys enter the room, stamp the snow off their boots and sit for a chat over beer and cigarettes, the Foo Fighters and German rap on the stereo. Every part of the cave around them is devoted to carnival. There are over 50 masks on the chipboard walls, animal skins and cowbells hanging from the rafters, jumbles of old mittens, massive leather belts.

‘All the costumes are handmade - you touch them and you can feel the hours of work that went into them,’ says Manuel. ‘Everyone brings their own unique ideas. I like that about Tschäggättä. It might be a monster or a witch or the devil or just a fantasy – when are you are a Tschäggä, you transform into another creature.’

It takes 90 minutes for that transformation to take place. Hoodies, tracksuit bottoms and trainers are replaced by sheepskin coats, cloth trousers and military boots, faces turning red as more and more clothes go on. The boys help each other make adjustments with a needle and thread or a strip of gaffer tape, and critically assess each look. It is a deadly serious business. ‘There are different groups in Lötschental and each wants to be the best. We almost don’t speak to the others when carnival is on,’ says Manuel, rifling through a rack of coats.

One of the final elements is the leather belt, into which each boy is manhandled in a process resembling some form of extreme medieval torture. They are lain on the floor and the belt pulled ever tighter round their waist by two others, feet on his stomach and chest to gain leverage. It’s a two-man job to hoist him to his feet again. With mask on and a broom, branch or umbrella to brandish, they are ready.

One by one, they emerge from the cave. Manuel jumps up and down to make sure his costume is secure, and waits for his band of monsters to assemble. ‘For Tschäggättä what you need is a cloudy night,’ he says looking into the sky. ‘Then it is mystical. You hear the bells in the distance and you wonder… where are they?’

At the end of the valley, in the village of Blatten, a giant Tschäggä statue has appeared, watching over people gathering along the route of the procession. In restaurants, friends gather to chat and warm up over mulled wine and fruit schnapps – but the atmosphere is still one of expectant terror. Just after 8pm, a flare goes up. The sound of screaming reaches me first, followed by the inevitable donging of cowbells. People start to run.

At first, a trickle of Tschäggä comes jangling up the street, each on the lookout for victims to menace. Within minutes, I’m surrounded by a good hundred – towering above the crowds, grunting and growling as they jog past. They push people around, pull their ears, hit them with sticks, drag them along the road. Some climb snowbanks and chuck ice over everyone below. Scarves and hats are stolen and tossed aside.

The procession runs for five miles along the valley, but spectators stick to the villages, leaving the Tschäggä to walk between them. I decide to follow for the whole route, and soon find myself alone, a small human surrounded by monsters, trudging through the night. Somewhere in the throng are Manuel and his boys, resplendent and freakish. As the lights and clamour of the villages fade, the Tschäggäs’ trot slows to a walk, their bells stop clanging and their sticks drag along the ground. I walk among them, hood up and shoulders hunched, hoping that in the dark and in their fatigue, they might mistake me for one of them. I make it to the final village, and leave them to their merry-making and mischief.

‘There are so many legends about carnival but we really don’t know much about the origins of it,’ says Thomas Antoniatti, who I meet the next morning, having dodged a lone, sleepless Tschäggä staggering about Wiler. The curator of the Lötschental Museum, which contains several ancient masks, one dating from the 18th century, his theory is that Tschäggättä is a mix of Christian tradition and baroque theatre – the church allowing a bit of controlled mayhem to keep the valley’s residents in check. ‘In the harshest moment of winter, you allow people to go completely nuts for a few weeks, and then you are into Lent,’ he says with a laugh.

In a back room of the museum, he shows me a copy of a letter written in the 1850s by Priest Gibsten of Kippel church. The earliest record of the carnival, it complains about the anarchy the Tschäggä cause. ‘He wanted to ban it,’ says Thomas, ‘but he had no success. The tradition is still living, and he is not.’ Just how alive that tradition remains is very much in evidence outside the museum, as the people of the Lötschental gather for the parade. If the Thursday night procession was the stuff of horror films, Saturday’s finale is pure slapstick. Brass bands march about, their members in multi-coloured suits. Decorated floats putter along, containing occupants dressed as nuns, polar bears and ninja-turtles, who toss sweets, confetti and lumps of cheese.

Among them are men, women and children in the customary Tschäggä costumes, the monstrous garb less intimidating under the bright sun. One zips past on a mobility scooter, waving a carpet beater. All find their way to the village sports hall, and sit at long tables scoffing beers, wine and cake. Prizes for the best monsters are announced, each awarded points for their costumes, build and character. Only when they are on stage collecting their prizes do they finally lift their masks and reveal themselves, to gasps and cheers from the onlookers. Boys from Manuel’s cave come second and third; a Tschäggä wearing Heinrich Rieder’s mask is first.

The drinking and celebrating continue long after the sun has set, costumes have been discarded, and fat snowflakes have once again started to fall from a sky turned the colour of an old bruise.

As I lie in bed that night, I listen out for bells, wondering if the Tschäggä are coming for a final time. It’s deathly quiet for a while, and then from somewhere down the valley comes that unmistakable ‘dong, dong, dong’.

I pull the sheets a little closer, and shut my eyes tight.