To start off Cornyx’s quest, you need to speak to him, in the Undead Settlement. Cornyx is a pyromancer of old.. and gives you the Pyromancy Glove, and sells all sorts of Pyromancy’s for your travels. He can also teach you new Pyromancy’s through Pyromancy Tomes. Head towards the Cliff Underside Bonfire, by jumping off the right side of the bridge you passed. From here, you want to follow the path that leads up to the roof, and take out any hollows that get in your way, if necessary. He is in respite, inside of a cage up here.
Go and speak to him, and he will warp himself, to Firelink Shrine. When he is in Firelink Shrine, speak with him to receive your Pyromancy Glove. Speak with him again, to recieve the “Welcome” gesture. Now, all that is out of the way… we need 3 Pyromancy Tomes. The first one, is in the Road of Sacrifices, .. and is the Great Swamp Pyromancy Tome, the elementary.. which nets you: “Bursting Fireball”, “Fire Orb”, “Profuse Sweat”, and “Poison Mist”. (Watch out for these critical foes on the way to the tome, these giant crustaceans really pack-a-punch!). Once your in the deep end, and acquire the tome, make your way towards Farron Keep, and take out the Abyss Watchers to grant access to the Catacombs of Carthus. Follow the path I’m taking here, to get to the Carthus Pyromancy Tome. Which yields: “Carthus Beacon”, “Carthus Flame Arc”, and “Acid Surge”. The area where the tome is, is behind an illusory wall, if you take the other route to it.
Now, the last tome we need is the Izalith Pyromancy Tome, which is near the Old King’s Ante Chamber Bonfire, in the Smouldering Lake. It gives you 2 Pyromancy’s from Izalith, which are the: “Great Chaos Fireball”, and “Fire Storm”. Once you have all the Pyromancy tomes that Cornyx CAN teach you, go back to Firelink Shrine, and give them to him. (Any other Pyromancy tome that is NOT SHOWN in this video, can only be taught from Karla). Cornyx originally sold: the “Pyromancer set”, “Great Combustion”, “Fire Surge”, “Fire Ball”, and “Flash Sweat”. (There is no special reward you get from buying all the Pyromancy tomes he has to sell, except the Pyromancy’s themselves).
All that is left, for Cornyx’s quest, is to acquire his set. To do so, you can either kill him, and buy it off the Handmaid in Firelink. .. Or, you can summon Great Swamp Cuculus, and have her surivive the Old Demon King boss fight. If she dies during the fight, simply save and quit, and re-summon her again. The only tip I have for this boss fight, and the only thing I think you would have trouble with.. is his AOE, when he goes down, and looks like he’s wounded. .. Don’t get greedy! When he’s downed, just wait for that AOE, then go in for the kill. After you have slain the Old Demon King, with Great Swamp Cuculus surviving the ordeal, .. she will do a bit of a victory roar.. before she vanishes into the mist. Don’t be rude. Wave to her goodbye.
Then, go to the Cliff Underside Bonfire, where you first met Cornyx, when he was contemplating in his cage.. And, on a corpse, just adjacent to the corpse where you can get the Hand Axe, will be Cornyx’s set, and the Spotted Whip. Sitting right outside of his cage. And, his blindfold can be found just before the Smouldering Lake, and just after the second Fire Demon, in the Abandoned Tomb. This concludes Cornyx’s quest. May the Flames guide thee. .
As found on Youtube