https://www.pcgamer.com/dark-souls-3-cut-content-suggests-pontiff-sulyvahn-was-once-the-final-boss/

Lance McDonald is a Soulsborne sleuth. After several months uncovering cut content from From Software's PlayStation-exclusive Bloodborne, the indie dev-turned-hobbyist game engineer has now turned his creative hand to Dark Souls. 

McDonald's latest discovery, revealed via a series of tweets, suggests Dark Souls 3 had players, at one stage in development, fighting Pontiff Sulyvahn and not Soul of Cinder in the game's final showdown. Moreover, the encounter was set in an unreleased version of Untended Graves, and not the Kiln of the First Flame.