You can combine a Silver Key with a Sealed Silver Box at the Chaos Goblin Machine to receive a Silver Box. A Silver Box is a reward box that can be tossed on the ground to receive a random reward.
The Silver Key is a P2P item, purchased in the cash item shop.
Elite Healing Potion (5)
Elite Mana Potion (5)
Cherry Blossom Flower Petal (1-3)
Cherry Blossom Wine (1, 2)
Cherry Blossom Rice Cake (1)
Jewel of Soul
Jewel of Chaos