As twilight falls across the snowy peaks of central Nepal, just over a steep ridge from the iconic Annapurna trekking trail, a herder scans the shadows with binoculars, searching for a lost yak. From a perch high above the tiny cliffside village of Nar, he spots a stealthy movement in a desolate meadow below the snow line. He sharpens the image. It's a young man, a stranger. Behind him is another. Eventually five more men creep into view, most still boys in their late teens, led by a man in his mid-30s. The herder knows immediately who they are and why they are there.

The Annapurna region is the ancient homeland of the Manang people; the intruders are from the Gurkha tribe, dirt-poor neighbors of the Manang in Nar. This ragged band of men has come 60 rugged miles to plunder Nar's riches: its fields of yarsagumba, a tiny, wrinkled fungus that is, by weight, the most valuable tonic in Chinese medicine. It has been prized for centuries as a potent aphrodisiac and elixir of youth; tradition holds that it will prolong virility through long winter nights and throughout a long life. On a good day, a yarsagumba picker can bag 400 pieces, gently yanking up the subterranean fungus by the gracile fruiting stem it sends up. He can then sell his harvest for as much as $1,000 – in a country where the average annual income is $500.

His lost yak forgotten, the herder quickly clambers down the scrubby hillside to spread the alarm. The village elders convene a hasty meeting to organize a posse. Mukhya, the communal law of the Himalayas, requires that one adult male from each of Nar's 63 households join the posse, so the guilt will be collective. During the night the men lay an ambush, surrounding the poachers' high, isolated position. As the mild June night wanes, the group's fury at the violation builds: They will do whatever it takes to protect their wealth.

At dawn a force of the fittest Manang youths storms the Gurkha camp, attacking with sticks and iron farming tools. The enraged Manangi beat two of the intruders to death on the spot and throw the bodies into a deep icy crevasse. They round up the other five and herd them down the mountain, where more men are waiting. There the beatings continue and quickly escalate. The angry mob rips the life from the poachers, bashing their heads with rocks. Every member of the group, including boys as young as 12, is required to strike a blow.

They then cut the five corpses into small pieces, wrap them in plastic, and throw them into a glacial torrent that carries them far away. Hands spattered with blood and splinters of bone, the 65 men and boys of the village sit in a mountaintop conclave and swear a solemn oath, promising never to tell anyone what they've done, not even their wives. It takes only a month for the murderers' pact to unravel and their secret to come out, as blood secrets always do.

Living in Asia, you get used to the impossible. I've lived in Bali for more than 11 years now, and most people in this part of the world believe in magical cures. If they get sick, they may seek Western-style medical treatment, but just to make sure, they also visit a faith healer, who dispenses the right herb or animal part to heal them. Of all these time-honored remedies, none is more sought after than those reputed to enhance male sexual performance.

Centuries before erectile dysfunction drugs, purveyors of traditional medicine were ransacking treasures of the wilderness for easy ways to stay hard. Some of them, such as seahorses and boiled swallows nests, are as innocuous as they are ineffective. Others are criminal: One of the principal causes for the continuing decline of the rhinoceros is butchery for its horn, said to resemble an erect penis. There are "medical" restaurants in China that specialize in serving animal penises to enhance virility, ranging from dog-dick sushi to tiger-penis soup. But the power of these elixirs is largely myth, arising from the primeval veneration of the great mammals, infused with their rarity.

Yarsagumba is by far the most precious of the Eastern aphrodisiacs. It's basically a mummified caterpillar that was once the larva of the Himalayan bat moth, which has been attacked by spores of the mushroom Cordyceps sinensis. Once harvested, the pale, delicate caterpillar carcasses are carefully kept whole until they reach the consumer's table, where they're crumbled into breakfast cereal or added to soup. Recent medical research has shown that unlike most natural aphrodisiacs, yarsagumba might actually work. A study at Stanford medical school found an increase of the 17-ketosteroid in the urine of men taking it, which indicates an increase in the subject's sexual drive and function and directly affects the sexual center of the brain and sexual organs. Controlled animal tests offer credible evidence that regular yarsagumba use decreases recovery time between orgasms and increases the volume of semen production. In another study on human subjects, 64 percent of Cordyceps eaters reported an enhanced sex drive.

In other words, Cordyceps doesn't work like Cialis – you don't pop one before a big date. To benefit from it, you must take it every day. Healthy Asian men of a certain age and income pay thousands of dollars yearly for a regimen of yarsagumba to hold on to what they've got – to cheat time. One ounce, comprising around 60 pieces – a scant one-month supply for a daily user – retails at urban Chinese apothecaries for up to $1,300, which means that it's literally worth more than its weight in gold. But these men are buying youth, the one thing even more precious than love.

It took centuries for the legend of yarsagumba to accumulate its power. A Chinese manuscript from a.d. 620 first recorded the tale of a charmed creature that metamorphoses in summer from animal to plant. By the 15th century, the fungus's reputation as an aphrodisiac was proverbial. A Tibetan monk named Nyamnyi Dorje Palden Zurkhar wrote a long poem in its praise called An Ocean of Aphrodisiacal Qualities, which promises that men who eat yarsagumba "will enjoy the delights of hundreds of thousands of beautiful women."

Yarsagumba first became known in Europe in 1726, when a Jesuit priest posted at the court of the Chinese emperor brought some specimens to Paris and presented them at a scientific meeting. It was even marketed in the United States in the mid–19th century by the Lloyd Brothers pharmaceutical firm in Cincinnati, but it soon disappeared from view except in traditional Chinese apothecaries, where it has always been venerated.

As the market for the fungus has grown, entrepreneurs have tried to cultivate it, but, so far, artificially incubated yarsagumba has lacked the mojo of the wild mushroom, which grows only in the alpine meadows of the Himalayas. Most of the yarsagumba harvested in Nepal comes from the Humla district, in the country's far north; undoubtedly, more prime yarsagumba fields await discovery.

Traditionally, the wealth of a village like Nar was its livestock, but in the early 1990s the economy of the Manang was revolutionized by the discovery of yarsagumba. Suddenly, simple farmers could earn more money in a few weeks than their fathers did in a lifetime. Every year in picking season, late March through June, itinerant Chinese merchants move in to sell the Manang people TVs and DVD players and set up impromptu brothels. While most of the fields in Nepal are in wilderness and open to all, Nar is so poor that it was given a special dispensation that allowed it to prohibit outsiders from picking in the mountains surrounding the village. Yarsagumba was supposed to save Nar.

A month after the massacre, life in Nar continued at its slow, pastoral pace, but the inner turmoil of the 65 murderers, all devout Buddhists taught to revere life, was as violent as their crime. The conspiracy began to fall apart when a delegation from Gurkha arrived seeking news of their missing kinsmen. They were brusquely turned away; nobody knew what they were talking about. Then a non-Manangi cowherd named Taghu Gurung Langzung took the group aside and told them where they could find the bodies of the victims.

The Gurkha visitors then went to Chame, the district capital 10 miles to the south, and told authorities, who dispatched a force of more than 60 policemen. They soon discovered the two bodies in the crevasse, decomposing in the summer heat and swarming with maggots. The police, well aware that mukhya required collective guilt, rounded up all the men in the village who were physically capable of having participated in the crime and herded them down the mountainside like sheep. It was the first time any Manang had ever been arrested.

Violent crime is so rare here that Chame didn't even have a jail; the authorities decided to transform the district education office into a prison large enough to hold the suspects. In the meantime they incarcerated them in a rustic lamasery nearby.

A curse had fallen on Nar: Women grieved; children were fatherless. The economy spiraled into entropy; untended, herds mingled together randomly until no one could tell which yaks belonged to whom; new houses were abandoned in the middle of construction; the hired hands, with no one to pay them, stole away by night. A village that had survived for centuries in one of the most extreme environments on Earth tottered on the brink of extinction.
The flight up to Manang was terrifying, as our Nepal Airlines Twin Otter turboprop skimmed like a well-aimed quoit through a deep V-shape gorge that was scarcely a hundred feet wider than the plane's wingspan. As the saffron-robed Buddhist monk next to me chanted under his breath and urgently clicked his prayer beads, I could make out the leaves on the shrubs I saw through the starboard window.

We landed with a bump at an airstrip in a village called Humde. It was a portal to a different world, chilly and pure, where the gaze is always drawn upward. White mountain peaks glistened as spidery drifts of snow caught in the updraft curled into flickering rainbows. A hand-painted sign on the hut that served as terminal read humde airport, manang. 11,000 feet. I was headed to Braga, a two-hour walk away, with Karma Gurung, a 26-year-old manager of a hotel along the Annapurna trekking circuit. He had been recommended to me as a reliable guide into the shadowy world of the area's yarsagumba trade.

"This wasn't the first time yarsagumba poachers were murdered in Nar," Karma told me the day before, as we sat in a dimly lit cafe in the Thamel district of Kathmandu, formerly the highest stop on the hippie trail, now a warren of alleyways lined with guesthouses and cheap restaurants that cater to a new generation of wanderers. "But before, they kept it a secret. The people in Nar are very dangerous.

"I used to collect it," he went on. "Most of my family is in the business. Almost everybody in Manang is involved." Karma explained that the yarsagumba trade in Manang started seven years ago, when yak herders noticed that animals grazing up near the snow line were healthier, more active – and hornier. "The people in Nar knew about it long before that," he said. "Everyone else wondered why they were up there with the yaks all the time. It was a secret then." Braga village sits at the foot of Annapurna, where yarsagumba grows in the alpine meadows.

On the outskirts of Braga, we arrived at the entrance to the protected area, where we stopped to show our credentials at a police checkpoint. When I flashed my press badge, the officer at the desk, a beefy man with a bushy mustache, asked me what I was writing about. I told him, and he smiled back broadly and said, "I am taking it." I asked him if it worked, and he gave his head a noncommittal Indian wobble. But after Karma and I set off and started climbing the trail, I looked back. The policeman shouted, "It works!" He lifted his right fist in the universal gesture of triumphant manhood.

As I traveled through the land of the Manang, virtually everyone I met was involved in the yarsagumba trade in one way or another. On the road to Braga, I met an intensely serious 10-year-old boy named Lakhba Tsering, who wore a leopard-print silk scarf tied over his head. He said his whole family picked yarsagumba. "On a good day, I can collect 400 pieces," he boasted. "I give it all to my father."

Old Braga village clings to the steep hillside like an Anasazi cliff pueblo, its mud-brick walls blending harmoniously with the terrain. The village is halfway between the airport and Manang village, the regional hub for tourism. In Manang they have Italian coffee machines and French wine for sale in the shops, but Braga retains a rustic feel. Karma's hotel, the New Yak, is situated just across a rushing creek from Annapurna III (elevation 24,786 feet), the third-tallest peak in the massif. In the morning the mountain looks close enough that a good shake would bring it crashing down on the hotel, and by early afternoon its shadow covers the village.

Karma took me to meet Samdu Tsering, a farmer from Phoo. Phoo is Nar’s sister village, but smaller and even more remote; the Nepalese usually refer to the area as Nar-Phoo. Like most people in the Annapurna, Samdu has seen his fortunes rise, following the trajectory of the yarsagumba trade, from extreme poverty to relative prosperity. Now he lives near an ancient lamasery in a snug, low-ceilinged house, dark as a cave and fragrant with juniper incense, which he shares with his daughter. "Everyone in Nar-Phoo collects yarsagumba," Samdu told me. "The women same as the men, and children as young as four." He said that children make the best pickers because they're closer to the ground and better able to see the spindly stem poking up from the dirt.

I asked him if he ever used yarsagumba himself, and he shook his head, looking slightly amused. Despite the intense romance surrounding the fungus, it's perfectly legal, more like the trade in saffron or orchids than that in narcotics, but Manang people like Samdu who harvest it rarely use it themselves, just as gold miners rarely wear gold jewelry: It's far too precious.

In fact, many people here have a moral objection to collecting yarsagumba. Later that night, over a dinner of dried yak meat, Karma told me that his cousin in Kathmandu had offered him 20 lakhs – nearly $30,000 – to run the trade in Braga, but Karma's father didn't want him to do it. "We believe it's bad karma to pick yarsagumba. It's like killing a living thing," Karma said. I pointed out that by the time it's picked, it's a mushroom, which possesses much less life force than the yaks we had just eaten. He shrugged and gave me a reply I couldn't match: "It doesn't matter if your belief is wrong – how can we know what is right? What's wrong is to go against your belief."
After another exhilarating and exhausting 16-mile trek, I arrived in Chame, the town where the men of Nar were being held. The next morning I met with the lead detective in the arrests, Inspector Bharat Bahadur Biswakarma. He was handsome in the clean-cut way of a soldier, with a jutting jaw and an intense gaze. Bharat wove a detailed account of the murders, showing me gruesome photographs of the two corpses found in the crevasse and letting me read the confessions he had extracted from the accused. He had some sympathy for them. "The Nar men only wanted to teach them a lesson, but as they beat them, it got to be more and more brutal," he said. The maximum punishment for murder in Nepal, premeditated or not, is 20 years in jail and the confiscation of property. When I asked him if he was shocked by the savagery of the murders, he quietly replied, "It was the worst crime that anyone here can remember."

Then I met the killers themselves. The former district education office is a two-story stone house that has been enclosed by a series of concentric barbed-wire fences wrapped around sharpened birch poles, giving the place the grim air of a gulag, but living conditions inside seemed to be comfortable. The men of Nar were sitting cross-legged in a lake of sunlight, playing board games and weaving harness straps for ponies they might never see again. They took turns playing badminton. Guards in camo fatigues stood in bored clumps, their .303-caliber bolt-action rifles, antiques dumped by the British after World War II, leaning next to them.

The portly jailer pulled a few chairs into the sunshine, and I sat opposite two of the accused murderers. The first was Karma Tashi, 26, whose full confession I had read in Bharat's office. He met me with a sullen, defiant face and denied everything: He wasn't there; he didn't know what happened. "I didn't do what it said in that statement. I don't know where the police came up with that."

The other prisoner who agreed to talk to me presented a more interesting case. Karma Wangdi Lama was only 17 and looked even younger, with a fresh, unformed face. "I was one of the men who climbed to the top," he said softly. In a quavering voice, staring nervously at the ground, he confessed his guilt. "I knew I was wrong, so I didn't run away. I waited for the police to come. I wish I had never done this thing. Before, I wanted to be a teacher. Now I think everything is over for me forever."

On my last day in Chame, word came by fax that 17 of the prisoners were to be released without bail on parole, to return for trial a month later. Among those released was a 21-year-old college student named Krishna Lama. He joined me for breakfast the morning after his release, but he wasn't celebrating.

"Yarsagumba brings a curse," he said. "Our entire village has had to suffer. Even my father had to face that fate." Krishna's father was murdered three years ago when he tried to drive away interloping yarsagumba pickers from Gurkha and southern Nepal. He, too, was beaten to death with sticks. Until last June, Krishna was attending college in Kathmandu, studying computer science with the money his father had saved from selling yarsagumba. He had just come home on a holiday to see his widowed mother when fate knocked at his door.

"I had to go," Krishna said. "A man from every house had to be a part of the group, and my father was gone." Krishna's story, that he was one of the last to arrive on the scene of the crime and didn't even witness the killings, is corroborated by the police, who obviously have sympathy for him. Yet under Nepalese law, as in most countries, his role in the conspiracy makes him culpable. He shrugged with a grim smile and said, "I am cursed. It happens to me time and again. I have no hope."
When I got home from Nepal, my friends all had one question: Did you try it?

Of course I tried it, though it was little more than a taste. A wholesaler I met in Kathmandu gave me a handful of broken pieces, which I washed down with tea. The crunchy little caterpillar fragments had a faintly fishy smell and went down with the musty flavor of most dried fungi. Forewarned, I didn't expect to feel a jolt of virility. In fact, I didn't feel much of anything at all, at first. It was only at the end of the day that I realized how much clear-headed energy I had after taking it. I had a glimpse of why people might come to believe in its power.

When Krishna Lama talked about the curse of yarsagumba, he didn't mean it as a metaphor. He was describing the fungus's inner power, which is more real to the people of the Himalayas than any scientific report. On the Annapurna trail, I met an old man who ran a little shop on the side of the road selling yak skulls and farm tools; he explained why devout Buddhists believe it's an evil power. "A famous lama chose to be reborn as yarsagumba as a boon to mankind," he told me. "If you trade in yarsagumba, you will be reincarnated very low. You won't be reborn as a man."

Like every gold rush in history, the yarsagumba boom in the Himalayas has blighted many more lives than it has enriched. As I write, the accused killers of Nar await sentencing, but it's almost certain that the terrible predictions of Karma Wangdi Lama and Krishna Lama are correct: Their lives are ruined. The legend of yarsagumba radiates an imperishable glamour, holding out the tantalizing promise of preserving the gifts of youth throughout life, but it's tainted by the blood of poor farmers and young men's dashed hopes. I'm not superstitious, but I know a curse when I see it.