The Black Pope

The Black Pope is a mythical being linked to the dark underbelly of the superstitious rumours involving the Roman Catholic Church. Rumoured to be a sorcerer dabbling in dark arts, a warlock in service to an archfiend, or a necromancer, he or even she is meant to live somewhere in or under the city of Rome. Some rumours persist the Vatican knows who the person is and it's below the Vatican itself the Black Pope lives. Others claim that this being lives in the Holy Land, usually around Jerusalem, or Bethlehem, and even tied to Constantinople. Others postulate the black pope is the diametric opposite of the Pontiff, and has the powers of Hell & the Devil at their command, like an evil cleric.
Very few details are known for sure about the Black Pope, though some things have been verified over time. Firstly, there has been a good number of Tieflings as the Plack Pope, and a small amount of chromatic Dragonborn. The hierarchy seems made up of a good few of these, as well as many humans. Their clothing is mainly dyed white for low level followers, purple and black for medium to high level members, and the Black Pope is usually in black coloured vestments. Amy trims or other adornments made of metal is made of silver. In quite a few istances the black pope and their hierarchy have been female.
The Black Pope & their followers have been having a covert war with the Christian religions for decades now. They sometimes back other major religions if it means Christianity loses, though this quickly turns sour & outright hostile when their erstwhile allies find out about the black pope and their backers, as well as their followers.
Sometimes the Black Pope is referred to as the Anti-Pope instead. Both titles can be interchangeable.

Summary

The idea of the Black Pope sprang up early in the Roman Catholic regions. With the constant hatred towards the Tieflings & the Dragonborn in Europe, those two dispossessed people found common cause to unite. At the time the Roman Catholic foundations, they decided to create an evil version of the Church hierarchy and with the caveat that whoever was strongest & wielded the most power bestowed by infernal fiends could act as the Black Pope. It did not matter if it was Tiefling, Dragonborn, male, female, young or old, as long as they could prove that they had the power to do so.
Dark magic, assassinations, recruitment of more of their kind and the formenting of evil cults throughtout Christian lands  was the order of the day. Wherever a powerful Roman Catholic nobleman or leader was to be found they would oppose them, and look to tear down everything that they stood for, primarly if it involved the Church. This led them to Rome where the oldest parts of the city was still tucked away below the streets & buildings built over the older parts. From there they sally forth on secret missions out of hidden ancient catacombs to carry out nefarious deeds.
The Black Pope has been linked to many other dults supplying information, rituals, and aid where possible while trying to find a way to truly bring down the Vatican. Leading truly blaspemous Black Masses from their hidden church below, they make a mockery of the sacraments & blessings of those they opose, all the while trying to find a way to bring all of it to ruin.
Exactly who this person is at the moment nobody can say, though there is the rumour that they have recently made great strides in the possibility of birthing an Antichrist, and are examining a number of sites throughout the world as to who the mother of the Antichrist is. The rumour ganed from the lips of captured followers point tp the child being fathered by the Black Pope themselves, and will be blessed with extraordinary power if successful.

Spread

The rumours of this is found throughout many Christian lands expecially where there is a strong Roman Catholic following.

Variations & Mutation

A variant of this is where a Roman Catholic grouping, such as a nation, diagrees with the choosing of a curent Pope and they choose a Cardinal of their own t become the Pope in their own stead. This rarely happens but this Pope is referred to as the Black Pope to others of the Faith.

Cultural Reception

Most do not believe in this and think it is just rumours and superstitious nonsence. Believers look upon it as a way by those of different beliefs trying to balance the books so to speak to say that if ther eis one, there has to be an opposing force. That, and a belief that something must happen to all the Tieflings & Dragonborn that survive into adulthood.