I would like to think this is Gwynn's Great Lord Greatsword reforged in the Profaned Flame by the Pontiff after his coupe against Gwyndolin. Given the resemblance to the Great Lord Sword, which is only differentiated by the hilt, which is often easily replaced over centuries, that's how I'm going to interpret it. After all, what was it before it became Profaned, since Profaned implies at one time it was NOT a Profaned/Defiled/Desecrated Great Sword, but one with sacred and reverential meaning? What other great sword would command reverence in Anor Londo? There is no better symbol for a New Order and Authority than taking the sacred weapon and symbol of the former Great Lord Gwynn and, like Gwyndolin's body being devoured and corrupted by Aldrich, remaking it in your own image. All the evidence I have is the location, Pontiff's campaign to corrupt/supplant/usurp/destroy Gwynn's family and former seat of power, and the implication that it could not be Profaned if it had not been sacred and treated as inviolate before hand. And only one weapon in the series would make a sensible candidate for such treatment: The Great Lord Greatsword. Which is, all in all, a weapon I quite liked, though not a winner for PVP purposes.
Then again, given how out of whack the Profaned flame is with everything in terms of Lore, take it as you like.