Direct answer: Use Cube Keys during the Version 2.2 Exciting Cube Unboxing event to progress missions and earn keys, Regular Hero Draw Tickets, and final Transcendence materials.
What are the official maximum rewards?
- Mission rewards: Cube Key x285.
- Mission rewards: Regular Hero Draw Ticket x10.
- Final completion rewards: Transcendence Engraving Stone x15 and Transcendence Refinement Stone x75.
Complete the Main Quest “HP Recovery!” to qualify. Cube Keys used accumulate toward event progress, and rewards must be claimed before the event ends.
Official source: A Joyful Moment - Exciting Cube Unboxing!.