You need to get to level 10 of the Volcano Dungeon on Ginger Island. You put the weapon in the Forge with a Prismatic Shard, click the button and spend 20 Cinder Shards. You'll get a random enchantment. If you want a different one, you just put it back in with another Prismatic Shard and spend more Cinder Shards... and keep repeating until you get the one you want. I recommend Crusader.