In order to obtain the 480 weapon, you need an item called the Perdurable Tomestone. And in order to get it, you'll need to trade in seven Blades of Golden Antiquity. These can only be obtained once per week by simply completing Eden's Verse: Refulgence.

Where can I find Allagan Tomestones?

Players can obtain Allagan Tomestones by completing dungeons, trials, Duty Roulette, and virtually every other endgame duty. There are two caps on tomestones: The current most recently-introduced type of tomestone has a weekly limit of 450 earned per week.

How do I unlock Eden's verse?

The Eden's Verse raid is unlocked in Amh Araeng after completing the previous Eden story. If you haven't completed the previous Eden raid then you will have to prior to taking on the new raid.