Board Thread:Dark Souls II/@comment-94.13.191.233-20150117094150

If you think about it, Old Dragonslayer is slightly larger than Ornstein and smaller than Ornstein's Super form, which could suggest that Smough's enhancements (size increase) from his death, passed on to Ornstein and remained there; which also suggests that it more than likely is Ornstein and that he did in fact survive the encounter with the Chosen Undead in Lordan long ago. But, back to the size comparison, if you think about it, when the player killed Smough first, Ornstein absorbed his power, and became equivalent to Smough in size; which could be what happened to Old Dragonslayer, it may have been that his power absorption slowly deteriorated over time, and instead of his lightning attacks, because they, too, deteriorated into darkness; which could have been a side effect of absorbing Smough's power.

Just a speculation. 