Pontiff Sulyvahn is a boss character and a major antagonist in Dark Souls III.
Young Sulyvahn was born and raised inside the Painting despite not yet experiencing loss or despair and had little use to his frost sorceries. As such, he left the painting and his belongings. Sulyvahn then discovered the undying flame of the ancient Profaned Capital. The flame gave Sulyvahn a newfound ambition to conquer the Boreal Valley, which the Profaned Flame was situated under, taking upon a greatsword bearing a flame similar to the Profaned Flame's. Upon fulfilling his goal, he formed his Outrider Knights to enforce his rule, even forcing his compeers to wear a ring that, should they ever question his ideals, would drive them mad with visions of battle unending and send them afar from the Boreal Valley in said madness. He turned all inhabitants of Irithyll and the Profaned Capital into slaves, soldiers, or beasts. He also gave his Fire Witches too much of the Profaned Flame's power, knowing it will drive them mad and turn them into mindless servants.
Sulyvahn would eventually institute worship of Aldrich, Devourer of Gods, into the city of Irithyll, taking the title of Pontiff upon it's formation. The Church of the Deep would regularly grant Aldritch sacrifices, to sate his insatiable hunger. Sulyvahn decided to further his grip over the Boreal Valley and sent Aldritch to devour Gwyndolin, the last of the gods in the old Anor Londo, and imprison Yorshka in a tower above her church.
The ashen one first comes across the Pontiff's influence in the Kingdom of Lothric when confronted with Vordt, one of his outrider knight's. Due to being harassed by the black eyes emanated from his rings, Vordt has transformed into a beast like and feral creature.
Later on in Irithyll, a city steeped in eternal night and snowfall the ashen one progresses towards Sulyvahn's cathedral and faces off against his knights (who have been reduced into mindless corpses) and sorcerers (whose hearts were consumed by the profaned flame after Sulyvahn introduced them to it) as well as the emaciated remains of the city's civilians. Upon reaching the cathedral the ashen one will fight the pontiff who dual wields a greatsword possessing the power of the profaned flame itself and a sword possessing a more sinister energy of the moon. Upon defeat, the ashen one is able to progress towards the Lord of Cinder, Aldrich, Devourer of the Gods.
The last of Sulyvahn's influence is manifested through the Dancer of the Boreal Valley encountered in Lothric who duel weilds a sword of magic and flame like her master. We learn that she was once a distant relative of the old Gods who Sulyvahn set out to destroy. The Pontiff had her enslaved as a dancer before making her an outrider knight who would lose her humanity and sanity.
Unlike most bosses encountered in the Souls series who have tragic backstories, Sulyvahn appears to have little reason to commit his atrocities apart from furthering his own ambitions. He usurps control over Anor Londo and has Gwyndolin, a God of the moon imprisoned and feeds him to Aldrich. The very fact that Sulyvahn appears to willingly work with Aldrich (it is somewhat implied that Aldrich is merely a pawn in the Pontiff's schemes himself) who is notorius for his gluttony and cannibalism, alone speaks volumes of his own moral compass.
Sulyvahn's minions are all under his command through the rings he gifts to his outrider knights, which turn them insane and beastlike. Or as shown through Sulyvahn's beasts, fear. This can be seen when performing a critical attack on them causing them to lay on their backs and "plead", implying that they are tortured by the Pontiff.
He is routinely referred to as a tyrant via NPC discussions and item descriptions and the state or Irithyll when the ashen one arrives further supports this claim. Whilst most of his actions occur before the events of the game, they can be heard of through item descriptions.