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Players would fight him in the Untended Graves area, which in the finished game was found behind a hidden wall after defeating Oceiros. The last boss you originally fought there was later renamed to "Pontiff Sullivahn" and moved to a different map, but Sullivahn was absolutely the last boss. Dark Souls 3 originally ended in the Untended Graves, but it wasn't the night-time version, there was 3rd version.
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