Beware the flame.
Dark Souls 3: Cornyx of the Great Swamp
The primary pyromancy trainer, Cornyx "Corny" of the Great Swamp is an essential vendor. Once unlocked, he awards the player a Pyromancy Flame, and can upgrade Pyromancy Flames as well as selling you new pyromancy spells.
Recruit Cornyx in Undead Settlement by letting him out of a cage and accepting his services.
Speak to Cornyx twice at Firelink Shrine to receive the Welcome emote.
Mysteriously, if you summon the NPC spirit Great Swamp Cuculus in Smouldering Lake and defeat Old Demon King, you can collect the Spotted Whip and Cornyx's Armour set from near the cage in Undead Settlement where you first met Cornyx.