Approaching the Firelink shrine bonfire after retrieving all the cinders allows you to kneel and receive the combined power of all the lords. This has no actual gameplay benefit but it does allow you to teleport to the kiln of the First Flame at which point you save the world by tossing a few more logs on the fire.
Or not. Seems like the Fire is guarded by the “Soul of Cinder”, which wields the combined strength of every hero to have ever linked the flame. That means it can literally do everything you can but better: Swordplay, pyromancy, sorcery and miracles.
Killing it doesn’t even end the fight. The boss just switches to a more godly fighting style full of fire and lightning and you have to kill it a second time.
The Soul of Cinder hits hard enough to blow through your guard and most of its attacks are sweeping 300 degree arcs that require near-perfect timing and positioning to dodge.
There is no gimmick. There is no secret. This boss is brutal even with a good build and I’m stuck playing as a Luck Knight.