With the Vorpal Sword and the Flamberge, you can complete the legendary sword.
Location of Origin
You can battle Origin at the obelisk in Ymir Forest after you receive the Vorpal Sword in Fenrir's Cavern and Flamberge from Odin's Tower. See Reach the Obelisk for directions.
Origin Boss Strategy
Attack with Lion Lunge, and when you can, get close to Origin and use Gale Shield repeatedly, backing him into the corner.
If you are lucky, you will win a ?Bow from Origin. It becomes a Soul Stealer, which is even better than the Elven Bow.
After the cutscenes are over, equip the Eternal Sword on Cless/Cress.
Now that you have the Eternal Sword, you can Reveal Dhaos's hidden lair.