Reforged

Reforge the special sword at a blacksmith's anvil once you've gathered what it needs (high-grade steel). You need the sword itself first - it comes out of the story - plus the smithing know-how and materials, so this is a later-game tick. Take it to an anvil, run the forging minigame and reforge it. Not missable; you can do it any time once you hold the blade.