Cybele, a mother goddess of Rome was at the center of a rather bloody Phrygian cult, and was sometimes known as Magna Mater, or "great goddess." As part of their worship, priests performed mysterious rites in her honor. Of particular note was the sacrifice of a bull performed as part of an initiation into Cybele's cult. This ritual was known as the taurobolium, and during the rite a candidate for initiation stood in a pit under a floor with a wooden grate. The bull was sacrificed above the grate, and the blood ran through holes in the wood, showering the initiate. This was a form of ritual purification and rebirth. For an idea of what this probably looked like, there's an amazing scene in the HBO series Rome in which the character Atia makes a sacrifice to Cybele to protect her son Octavian, who later becomes the emperor Augustus.
Cybele's lover was Attis, and her jealousy caused him to castrate and kill himself. His blood was the source of the first violets, and divine intervention allowed Attis to be resurrected by Cybele, with some help from Zeus. Thanks to this resurrection story, Cybele came to be associated with the endless cycle of life, death and rebirth. In some areas, there is still an annual three-day celebration of Attis' rebirth and Cybele's power around the time of the spring equinox, called the Hilaria.
Like Attis, it is said that Cybele's followers would work themselves into orgiastic frenzies and then ritually castrate themselves. After this, these priests donned women's clothing, and assumed female identities. They became known as the Gallai. In some regions, female priestesses led Cybele's dedicants in rituals involving ecstatic music, drumming and dancing. Under the leadership of Augustus Caesar, Cybele became extremely popular. Augustus erected a giant temple in her honor on the Palatine Hill, and the statue of Cybele that is in the temple bears the face of Augustus' wife, Livia.
As the Roman Empire spread, deities of other cultures found themselves absorbed into Roman religion. In the case of Cybele, she later took on many aspects of the Egyptian goddess Isis.