From various datamined information and really early marketing images, Dark Souls 3 originally seemed to have a very different story with Sulyvahn at the centre. He is called "Old King of the Eclipse" in the game code, and was going to serve as the game's final boss in a dark version of the Untended Graves.

Somewhere during development they shifted the story back to linking the flame, making Soul of Cinder the final boss in a distinct version of the original Kiln of the First Flame, demoting Sulyvahn to a mid-game roadblock, and reusing the dark Untended Graves as a secret area where Champion Gundyr is found.

It's easy to see the original story plan in the game though, much of the primary content is related to or driven by Sulyvahn, such as the state of the Dancer & Vordt, the wandering Boreal Knights, stealing the Profaned Flame from Yhorm's city which he wields during his fight, and even Gwyndolin being fed to Aldrich to keep him distracted from Sulyvahn's seat of power in Irithyll, which is where you find him. The Ariandel DLC is even connected, revealing that Sulyvahn originated from inside the painted world.