The Moon

Complete the Core to unlock the Moon, the deeper area on the Crater branch of Act 2. The Moon is a surreal, alien environment with unique enemies and the Man in the Moon boss. Clearing the Moon is one of two requirements (along with the Core) to reach Act 3. The Moon also plays a role in unlocking the Rift secret area, as you need to sacrifice a cat holding the Scalding Orb to the Man in the Moon.