With the Vorpal Sword and the Flamberge, you can complete the legendary sword.
Lake of Ymir
Equip the Flamberge.
After Arche leaves, make sure to equip Chester and put him behind Cless/Cress in the formation.
Go to the Elven Village, then go to Treant's forest. To reach the obelisk, go north three times, east twice, north twice, east twice, south twice, then through the southeast exit, and east.
Get ready to fight Origin. First, set Cless/Cress to use Gale Shield, Lion Lunge and Mecha Blade. Set Klarth/Claus to call many spirits but only use Volt, and equip him with the Sapphire Ring and a Mystical Rune.
Go up to the obelisk to fight Origin.
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 cut scenes are over, equip the Eternal Sword on Cless/Cress.