The victims of witch hunts face gruesome punishments, according to “Contemporary Practices of Witch Hunting,” a 2015 report by the Indian legal nonprofit Partners for Law in Development. They can be subjected to “forcible stripping,” the report said, “being paraded naked in public, cutting or tonsuring of the hair, blackening of the face, cutting off of the nose, pulling of the teeth to ‘defang’, gouging out the eye, whipping, gang rape, forcible consumption of human excreta, cow dung” or “killing by hanging, hacking, lynching or burying alive.”

Ms. Rabha traveled from village to village in Assam to speak out against the practice and declared that there was no such thing as “daini,” or witches. She had long been suspicious of folk superstitions and of medicine men who chanted incantations over young women to drive out what they believed were evil spirits. As a young mother, Ms. Rabha was told by a local medicine man that her mentally ill son would soon die; he didn’t. That false prediction, in the 1980s, was the seed for her advocacy work, which she began in earnest around 2000.

That year, she stood up in a meeting in the village of Lakhipur, also in Assam, to support five women accused of being witches; she didn’t back down when hundreds of villagers surrounded her house the next day.

Usha Rabha recalled her first rescue mission with Ms. Rabha, in 2006, when a stick-bearing mob encircled them in a neighboring state. “I was terrified,” she said. But Birubala was “completely unfazed,” Ms. Rabha said. When the police came to rescue the two women, she said, Birubala “reprimanded the cops, saying, ‘I will not stop until I finish the work that I do.’”

In Assam, in the 2000s, Birubala Rabha became allied with the state’s former police director general, Kuladhar Saikia. “She would come to me, meet me and discuss these issues,” Mr. Saikia recalled in an interview, adding, “She told me she was standing up against social injustice.”