Jeffrey Vaughn, content designer for Arenanet, dropped more information on the Final Rest weapon on the forums today.
I really can’t give out any hints or information, especially since anything I say is going to be intensely analyzed. So I’m going to play it safe and not comment on the size of the boss, the chest, or the rarity of the boss. I will confirm that I looked in the game data and traced the item to its loot table to the chest that uses the loot table to the event that spawns the chest to make sure everything is set up. I can also confirm that it has a very low drop rate, so running the event 5 times or 10 times proves nothing.