Instructions
Hunt down and defeat the "Giant Crab" boss in the River Belle Path area. You will need to get at least 150 bonus points in the battle, then select the "Ancient Sword" from the spoils menu. You need this item to craft the Ragnarok. If you are unable to select the sword, you will need to defeat the boss again when it regenerates.
Locate and kill the "Goblin King" boss in the Goblin Wall area. You will need to get at least 200 bonus points to have a chance of selecting the "Cursed Crook" item. This is another item you need to forge the Ragnarok. Again, if you are unable to choose the Crook after the battle ends, you will need to defeat the boss again when he regenerates.
Complete the "Rebena Te Ra" quest with at least 180 bonus points, then select the "Dark Weapon" design from the spoils list. This is the design needed to forge the Ragnarok.
Travel to the nearest Blacksmith shop and speak to the smith. Ask him to create an item for you, then select the "Ragnarok" from the list. The smith will forge your Ancient Sword and Cursed Crook into the Ragnarok sword. This weapon is only usable by Clavats.