Picture this: barefoot men and women in traditional dress, dancing over glowing embers, their faces blank and inward-looking, while a drum and a bagpipe repeat a simple, hypnotic melody that seems to have no beginning and no end. The crowd around them is silent. The night sky above is thick with stars. The embers are very much alive.

This is Nestinarstvo, or firewalking, one of the most arresting sights Bulgaria has to offer and, if you are willing to travel for it, one of the most rewarding. You may have encountered a version of it already: trained performers do a tourist-friendly rendition at Black Sea resorts and in certain restaurants in Sofia. Those are entertainment. The real thing happens once a year, on the night of 3 June, in the village of Balgari in the heart of the Strandzha mountains, in Bulgaria's southeast. It is a different experience entirely.

Balgari is the only place in Bulgaria where the Nestinari tradition has been preserved as a living community rite rather than a performance. The event on 3 June – the feast day of Ss Constantine and Helena, the patron saints of the firewalkers – draws thousands of visitors. Plan ahead. Book accommodation in Malko Tarnovo or one of the surrounding villages well in advance. On the day itself, expect police checkpoints on the approach roads, cars parked for kilometres along the verges, and a lengthy walk to the village. Arrive early to secure a place near the fire circle; in recent years a large screen has been installed over the village square for the overflow crowd, which gives you a view but not an experience.

The atmosphere on arrival will be familiar to anyone who has attended a Bulgarian village fair: food stalls, the smell of grilled meat, folk music from loudspeakers, children clutching cheap toys. It can feel incongruous. Persevere. As dusk falls and the bonfire that has been burning all day is reduced to a wide circle of embers, the mood shifts.

The Nestinari icons of Ss Constantine and Helena are revered and kept year round in a dedicated chapel. Believers treat them as living beings: they clothe them and, during major festivals, take them to sacred springs for a bath

The ceremony actually begins before 3 June. On the Sunday preceding the feast, the Nestinari icons – images of Ss Constantine and Helena, dressed in cloth as if they were human – are carried in procession from their usual abode, a humble chapel on the edge of the village square to a place deep in the wilderness called Vlahov Dol. There, at springs believed to have healing properties, the icons are ritually bathed and then returned to Balgari. The procession is repeated on the morning of 3 June.

By evening, the bonfire in the square has burned down. A man with a rake arranges the embers into a large, flat circle. Inside the chapel, the Nestinari are meditating – they believe that St Constantine himself enters their hearts and minds, providing the guidance and strength they will need. Outside, the bagpipe and drum have already started their relentless, trance-inducing rhythm.

When Nestinari head for the embers, they follow a small band of musicians and young men carrying icons of Ss Constantine and Helena

Then, out of the darkness, the Nestinari emerge. They stand at the edge of the fire circle for some time, praying, swaying, going deeper into themselves. One by one, they enter the embers – barefoot, eyes elsewhere, some carrying an icon held aloft. They dance in and out of the fire, sometimes crossing the full width of the circle, for about half an hour. Then, as if at the signal of some invisible conductor, it is over. The Nestinari take the icons back to the chapel. The embers are still glowing. Inevitably, some members of the audience pull off their shoes and try their luck. Many are burnt.

The Nestinari are, by all credible accounts, not burned. Researchers have proposed that the secret lies in technique: the dancers take quick, shuffling steps that reduce the time any part of the foot is in contact with the embers and limit the oxygen available to fuel burning.

The Nestinari themselves have no interest in such explanations. They believe the saints protect them. That belief is, for them, both the reason they enter the fire and the reason they emerge unharmed.

Preparing the embers

No one knows precisely how old Nestinarstvo is, or where it began. The earliest written accounts date only to the 1860s, and later ethnographers who tried to research the rite were met with deep reluctance – the Nestinari did not discuss their beliefs with outsiders. What is generally agreed is that the tradition is far older than Christianity, most likely rooted in Thracian religious practice.

The leading hypothesis connects it to the cult of the Great Goddess and her son and husband, the Great God – a divine couple who, in Thracian belief, had created the universe together and manifested as Light and Darkness. When Christianity reached the Strandzha in the 5th or 6th century, these ancient figures did not simply disappear. They merged, quietly and pragmatically, with a Christian pair who seemed to fit: Emperor Constantine the Great, who institutionalised Christianity in the Roman Empire, and his mother Helena. People in the Strandzha traditionally believed – against the historical record – that Helena was in fact Constantine's wife, which preserved the older mythological pattern of the divine couple.

The rite was never exclusively Bulgarian. Until the early 20th century it was practised by both Bulgarians and Greeks across several villages in the Strandzha, celebrating a range of spring and summer religious festivals. When the Balkan borders shifted after 1912-1913, the Greek population of villages such as Kosti and Brodilovo departed, carrying their Nestinari icons with them. Their descendants now live in northern Greece, where they still perform their own version of the rite, known as the Anastenaria.

First steps into the fire

In Bulgaria, the 20th century was not kind to the tradition. The departure of the Greeks removed a significant part of the community that had sustained it. Then came Communism, and with it a systematic campaign against what the authorities dismissed as dark superstition incompatible with a rational socialist society. The number of practising Nestinari dwindled steadily. The last of the old generation died in the 1960s.

Revival came from an unlikely quarter. In the 1970s, Communist Bulgaria grew more nationalistic, and Lyudmila Zhivkova – daughter of party and state leader Todor Zhivkov, and a woman with a pronounced interest in mysticism and the occult – championed a renewed enthusiasm for Bulgarian folk heritage. The Nestinari were rediscovered: their rite was spectacular, visually striking and easily packaged as a tourist product. Trained firewalkers began appearing at the large Black Sea resorts, performing for foreign tourists. A tradition that had nearly died became, paradoxically, a brand.

After the collapse of Communism, the Balgari dances were revived. In 2009, Nestinarstvo was inscribed on the UNESCO list of intangible cultural heritage – recognition of its uniqueness, though also, inevitably, another step towards its institutionalisation.

It would be dishonest to pretend that what takes place in Balgari on 3 June is unchanged from whatever the original rite was. The depopulation of the Strandzha over the past century, the loss of the Greek communities, the Communist interruption and the subsequent tourist revival have all left their marks. The modern ceremony differs from the original in ways that are often significant and has gradually morphed into a spectacle.

And yet. When the embers are raked flat and the bagpipe and drum begin their loop, and the Nestinari emerge from the dark and step into the fire under the stars above the Strandzha, something genuine is still happening. Whether it is a memory of a Thracian rite, a Christian ceremony, a community's act of collective will, or something that resists all these categories, it will stay with you.

It will also leave you with questions that have no easy answers. Where exactly did this begin? Why fire? What is the relationship between the Strandzha tradition and similar fire-walking rites documented in ancient Rome, the Middle East, China? Is it a form of shamanism? A cure for psychological pain? A technology of trance that human beings stumbled upon independently across the world? The Nestinari will not tell you. That, perhaps, is the most authentic thing about them.