It is a mix of Islam with earlier Hindu, Buddhist and traditional beliefs — something that is typical in Indonesia, and especially Java.
"Indonesia is amazing because whatever religion comes into Indonesia, it changes its color," says Kunijoro Soeparno, a professor of sociology who’s been researching these traditions for decades. The Catholic Church is different from in Rome," he says.
In spite of that, though, the ritual on Gunung Kemukus is exploding in popularity.
There are lots of women standing by themselves and lots of men in groups. He is balding, has a moustache, wears a leather jacket and glasses, and is reading from the Koran.
He's married, but hasn't told his wife he is here because she wouldn't let him come. "In all the rooms around here there are couples," he says.
I am keen to do the ritual; it's just a matter of finding someone who wants the same thing." Like tens of thousands of others, Slamat has come to be blessed in a ritual that dates back hundreds of years.
The ritual's origin is a legend about a young prince, Pangeran Samodro, who was raised in the court of Demak, a Muslim sultanate on Java’s north coast, and fell in love with his stepmother, Nyai Ontrowulan.