From previous FF games (ignoring Apocalypse, Ragnarok and Liberator)

There is Masamune, although technically a Katana you have a great sword design that Auron used in FFX and rikku as DRK in FFX-2. (in FF3 DRK and NIN both used darkblades of which the Masamune was one)
If you argue that SAM (if added in future) should have it, there are still quite a few alternatives for SAM like the Amanomurakumo or Kogarasumaru.

More likely, since there is already similarities to Cecil's DRK from FF4 and it being a weapon given to DRK after completing the additional part of the starter quest for the job in FFXI, it would be the Deathbringer.