Carthus was once a violent land filled with pyromancers. The warriors that existed there were considered to be very powerful and used curved weaponry. The Carthus swordsmen were very pragmatic, and unlike “noble” knights, they attached all glory to victory. There was no “dishonorable” path to victory, after all, the dead kept no honor.
Carthus warriors did not only use pyromancy to destroy the weapons and armor of their foes, they also used it to improve themselves and their fighting styles. Sometimes it was by infusing their blades on fire using the Carthus Flame Arc pyromancy. Other times it was to improve the power of their blows. From the Carthus Beacon pyromancy, which also describes their warlike nature. The pyromancies of Carthus are noted to be different than others. This is explained by the Carthus Pyromancy Tome as being due to their “being developed in isolation”.
At one point, all of Carthus would be united under one high lord. This was Wolnir, a paranoid and deadly man, who did not care for the Gods. Wolnir was able to command the complete loyalty of the Carthus warriors, and seems to have been a very successful conqueror, as he conquered most Kingdoms known to his people. He was paranoid ruler who sentenced many to their deaths. But all men’s rule must come to an end, and Wolnir’s was no different. Eventually, he was swallowed by the Abyss, which caused him to seek comfort from the Gods for the first (and last) time. I don’t know what caused this, but it could have come about due to Wolnir’s actions, or perhaps he dug too deep for power. In any case, Wolnir took a holy sword and armlets stripped from clerics as comfort. After Wolnir fell to the abyss, he discovered something in there, the black flame. His former soldiers would become Grave Wardens, and learn more of the forbidden flame.
The first black flame pyromancy discovered seems to have been the Black Serpent Pyromancy, as it inspired the other grave wardens, and continues the idea of the abyss having its own will. Other flame arts discovered by the Grave Wardens include the Black Fire Orb and Black Flame, which for the most part state the same information, which is restating that they were discovered after Wolnir’s fall. Eventually, you meet Lord Wolnir after somehow being pulled inside of his skull. Once there, he seems gigantic, which might just be his representation, as the skull that pulls you in is human sized. There, Wolnir commands the ghosts of his former warriors, and by shattering his armlets, the symbol of his comfort, he disperses into the abyss.