its been a while so i could be wrong. iirc in gefenia at the area where u fight the elder sister there should be an abandoned house at the top left of the map. u need a key from the elder sister to open it though. should be a chest with 3 broken swords in it. after that bring it to the blacksmith and he can repair it if u bring the materials
Chiming in on this, that seems to be correct or close to correct. I remember doing something like that from my playthrough when translating this.
I believe it's in the last to second to last layer of Gefenia or whatever that final area is called. I believe it's tied to some side quest, so if you can't open the door to that abandoned house in that forest right before where you fight Mysteltainn, try to do all the sidequests and/or speak to the Guild Receptionist for more.