Never heard of such rites for Mithraism. There is a bull involved but it is the sacrifice not a person. I think you’re confusing Mithraism with this.
https://en.wikipedia.org/wiki/Brazen_bull
Mithraism
https://en.wikipedia.org/wiki/Mithraism#Degrees_of_initiation
Phalaris, the despot of Acragas (modern Agrigento in Sicily), was a heartless, sadistic tyrant renowned for his imprudent cruelty; his alleged atrocities include devouring infants. One day, Perillos of Athens told Phalaris of his newest invention, a torture and execution device that would strike fear into the hearts of his enemies: The Brazen Bull.
The Brazen Bull was a device with a simple concept but a downright diabolical execution. Made entirely from bronze, it was the same shape and size as an actual bull, and housed a hollow chamber in its body. The poor person set to be executed by the bull was locked inside of this chamber, then a fire pit was set under it, heating the belly of the beast until the victim inside roasted to death.
A countryman of my own, one Perilaus [another rendering of Perillos], an admirable artist, but a man of evil disposition, had so far mistaken my character as to think that he could win my regard by the invention of a new form of torture; the love of torture, he thought, was my ruling passion He opened the back of the animal, and continued: When you are minded to punish anyone, shut him up in this receptacle, apply these pipes to the nostrils of the bull, and order a fire to be kindled beneath. The occupant will shriek and roar in unremitting agony; and his cries will come to you through the pipes as the tenderest, most pathetic, most melodious of bellowings. Your victim will be punished, and you will enjoy the music.
As if the Brazen Bull wasnt brutal enough, it was designed to channel the victims screams through a series of pipes and out of a set of holes on the bulls face. This gruesome acoustic apparatus transformed the desperate screams so that they sounded like a bulls angry bellowing.
Reportedly, Perillos told Phalaris His screams will come to you through the pipes as the tenderest, most pathetic, most melodious of bellowings. Appalled by these words, the tyrant ordered the acoustic system to be tested on the creator himself and pushed Perillos inside the bull. The doomed craftsman was locked inside and the fire was set, and soon Phalaris could hear the sound of his screams. Before Perillos could die, the tyrant opened the locked door and took him away. Perillos thought that he would be rewarded for his evil invention, but instead, Phalaris threw the unfortunate craftsmen from the top of a hill.
In an ironic twist, is said that the tyrant Phalaris himself died in the Brazen Bull when he was overthrown by Telemachus, the ancestor of Theron.
https://www.thevintagenews.com/2016/10/10/brazen-bull-ancient-greek-torture-device-considered-one-gruesome-creations-time-2/