In the Lost Cavern Event, you need to complete unique levels and earn points to get the rewards.
There are three milestones with a different number of core levels. When you reach them, you get a chest-reward.
IMPORTANT! If you lose a level or close it without completing, your progress will reset to the last milestone reached or a starting position.
The Lost Cavern is a local event. Actions made in unique levels do not count in Global events, nor do they bring you any Stars or Gold.