because you can't legitimately kill Gwyndolin before fight Ornstein
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No, you can kill Gwyndolin before fighting Ornstein if you get the Darkmoon Seance Ring from the Catacombs. But that doesn't make a difference in telling whether or not he is one of Gwyndolins illusions, since Gwynevere doesn't disappear if you kill Gwyndolin -> his illusions don't need him to be alive to stay in effect. Ah, but I guess it has to be the real Ornstein since he drops both his soul and his unique ring (item description: "The Leo ring belonged to Ornstein the Dragonslayer.").
