Lore
Born in the Painted World of Ariandel, Sulyvahn experimented with sorceries from a young age, several of which can still be found there. He left early in his life, as, having not come there by choice, the rotting world for hopeless souls held no appeal for him. As he progressed through the lands outside, however, he discovered their own flaws, including the stagnation which mirrored that of his homeland. A grand ambition grew within him, and as he travelled he began to amass followers and grow his power. His Profaned Greatsword is imbued with the Profaned Flame found in the Capitol below Irithyll, and the Fire Witches in his army and Jailers he appointed to his dungeon hail from there. It is implied that he travelled too to the Kingdom of Lothric, where he may have advised the Princes, turning them away from their appointed path to link the Flame. In his other hand he weilds the Greatsword of Judgement, a weapon meant to represent loyalty to the Covenant of the Darkmoon, but secretly imbued with Sorceries, which are seen as heretical to the children of Gwyn. He likely served under this covenant falsely while gathering his powers before staging a coup and appointing himself Pontiff, another word for Pope. He was directly connected to Aldrich and was the one that imprisoned Gwyndolin to be devoured by him, implied to have weakened or poisoned Gwyndolin somehow to prepare for this. As Pontiff of the Church of the Deep, he is supported by Aldrich's forces, seemingly working as partners in their efforts to destroy the Flame and usher in the new Age of Deep, though Sulyvahn's true motives are never made clear beyond his lust for power.
Pontiff Sulyvahn bestowed a special ring upon his knights. The rings granted them boons in battle such as increased strength, but slowly transformed them into twisted beasts, ensuring that they could never turn against him while not directly under his watch. These rings were only granted to those knights who were dispatched to foreign lands, called Outrider Knights. Two in particular we face are the Dancer of the Boreal Valley and Vordt of the Boreal Valley. The monstrous Sulyvahn's Beasts which are found in several areas surrounding Irithyll are also his past Outriders, malformed but ever loyal, who managed to return home after their transformation, guarding him still.
Though motivated and apparently charismatic enough to have amassed such an army, Sulyvahn is also cruel and cunning, having been able to grow close enough to Gwyndolin to successfully betray him as well as twisting and distorting the remaining citizens of Irithyll into his slaves and prisoners, bent on carrying out his will at any cost. Dogs in Irithyll are different from those in other areas, with almost human skull-like faces, implying that they too may have been victims of his dark magic. Beyond this, the Dancer's armor description marks her as a descendant of the family of the Old Gods, such as Gwyn, and his particularly cruel treatment of her by first forcing her to become his enslaved entertainer and then his knight, indicates a disdain not just for Gwyndolin but the royal family on whole. Oddly, despite this, he has trapped Company Captain Yorshka in a tower rather than killing her or feeding her to Aldrich.
In the end, he nearly succeeded in his goal of eradicating the current age, until the Player Character arrives.
Theories
A statue shows the face of a younger Sulyvahn
The Crying Tree in The Painted World of Ariandel is Sulyvahn's Mother
Sources
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