Octo’s Treasure chest is full of rewards. To claim them, first, you will need to be able to complete five levels in a single life. When the event is ongoing, you can enter it by tapping the Octo’s Plunder button on the map screen, and a mini-map will open. Complete the levels on the mini-map with a single life, and the treasure is yours! Once you have completed one round and claimed the Treasure Chest, you can try completing the levels again to get a second reward.