The Cult of Cybele
This cult is based around the mother-goddess Cybele, who came to Greece around the 5th century, and was always depicted sitting on a mountaintop, surrounded by animals and playing the tambourine. So, to worship her, her followers were all about the loud, percussive music and frenzied dancing. As well, they’d participate in something called the Taurobolium, a ritual bull slaughter that involved anyone getting initiated into the cult to position themselves below the bull and showering in its blood. But that’s not all. It gets better.
Later on a new dimension was given to the Cybele mythos with the absorption of another cult; the Cult of Attis. According to mythology, Attis was a mortal who spurned the goddess Cybele’s advances and was punished with madness, causing him to cut off his testicles and die. Later, Cybele regretted her actions and petitioned Zeus to allow Attis to be resurrected.
As a result, all the priests of the Cult of Cybele did the same thing – publicly cut off their balls in the hopes of resurrection. Supposedly.