I searched the wiki on Great Swords and Bastard Swords and couldn't find a match. It could be this is an NPC-only item. However, as it is equipped on a PdK there is one final possibility that isn't easy to check. In Wheloon and possibly Stormhorns too, I am uncertain, Wondrous Craftsmanship Items sometimes drop. These items were given unique appearances. But because it is so random... when one drops, what type of item drops, etc. IIRC they only drop in named item chests in Wheloon and possibly Stormhorns and on the chain end reward lists to those two quest chains. As they are so rare and it could be any weapon or accessory that drops I don't believe there is much documentation of the unique appearances of these items. There is the slimmest possibility that this could be a Wondrous Craftsmanship Bastard Sword or Great Sword. I have kept a few of these items but unfortunately those weapon types are not in my small collection.