After the last big update in the world of Sphere 3 there appeared new caskets of item upgrades that you can loot from the defeated enemies. To open the casket and get its contents you will need a special key!
You can buy these keys in the special section of the Shop.
Depending of the type of the casket you may need copper, iron, silver, golden or platinum key! You may see the list of the casket contains in the key description.