Taurobolium

During the Roman empire, the taurobolium referred to practices involving the sacrifice of a bull, which after mid-2nd century became connected with the worship of the Great Mother of the Gods; though not previously limited to her cult, after 159 AD all private inscriptions mention "Magna Mater" (the Roman name for the goddess Cybele). Originating in Asia Minor, its earliest attested performance in Italy occurred in 134 AD.
Ritual.
At the taurobolium ritual, the high-priest would stand inside a pit made specifically for the purpose of the ritual, a bull would be led above the pit and sacrificed above him, the blood of the bull would pour down onto the priest, showering him in the blood. After this, the bulls testicles were removed and taken to the sanctuary as an offering. This ritual was done as a replacement for the castration of high-priests, this was because castration of male Roman citizens was forbidden in Rome.