A bizarre story of an affair between a runaway young prince and his step mother who were caught, killed and buried sounds like an x-rated Disney movie, but instead it's a myth that has led to thousands of Indonesians having sex with strangers in the hopes of getting some good luck.

An ancient ritual dating back to the 16th century, thousands of people – many of whom consider themselves devout Muslims – flock to the Gunung Kemukus, otherwise known as 'Sex Mountain' in Central Java to find a partner.

Dateline journalist Patrick Abboud travelled to the mountain, and discovered that legend dictates an individual must have sex on the mountain every 35 days, seven consecutive times, in order to receive blessings and wealth.

However, pilgrims cannot undertake this ritual with their spouse; they must sleep with someone outside of their marriage. They must also first make an offering at the grave of the young prince, pray, and wash at nearby sacred springs.

"You won't see this ritual anywhere else in Indonesia or the rest of the Muslim world. It's a very Javanese blend of religious ideals – with Islamic, Hindu and Buddhist influences," Abboud says, adding that he was "shocked" by how busy the mountain was: "Thousands of people, up to 8,000 on a busy night."

Legend has it that the prince and his lover were caught mid coitus, and therefore "Because they didn't get to finish the act, others believe that if you do, fortune will come to you," Abboud explained. "It's a bit of a carnival."

As a Muslim-majority country, the practice contravenes many of Indonesia's strict laws. However, the government turns a blind eye, and even makes money from pilgrims by charging an entry fee to the mountain, and has built 'sex shacks' for people to use; charged by the hour.

It's become a site popular with prostitutes, so these unable to find a partner can purchase sex instead.

The official policy remains that it is a holy site, however. A gatekeeper told Abboud: "Pilgrims should come here with pure hearts and clean bodies. We've never said the sex is a condition of the pilgrimage. It's what they want to do."

Professor Keontjoro Soeparno, a social psychologist from Gadjah Mada University in Yogyakarta, told Abboud that the government has "facilitated the rise of prostitution."

"The Islamic religion forbids all this, but the government would rather not know about that. Because they're more interested in profit – they leave their religion behind," he said.

However, Abboud told the Daily Mail that the ritual should not be used to "demonise" Islam.

"It's not strictly Islam as we know it," he said. "'You'll never find any ritual like this in any part of the Muslim world and Indonesia, Javanese interpretation is much more liberal."