Hey there. You need to first earn the Legendary version of the weapon by opening all Region Chests and finding a collectible within each Cyst. You need to get the collectible from the Sword Dance cyst and Aerial Ace. That will let you claim the Legendary version from a chest in the Divide. To earn the Exotic, you need to find all 8 Visions of the Traveler and defeat all 9 Overthrow bosses. You've already done those, but the chest can't be opened until you've also earned the Legendary Khvostov too.