You need to go to Teiagre Castle, Do the knights task. Then, as soon as you enter Teiagre Castle, you should see a knight standing in front of a market, Very similar to the market you see when you buy the dagger and the market you see briefly in the ending.

Okay, So now all you must do is simply press E on the market, And you should have the sword.

Congratulations if you did everything correct and you have the sword, Tell me if I made any mistakes while making this guide and I will correct them. Thank you and have a good day to you all.