You must navigate to the upper-most parts of the cave system. You'll need to overcome obstacles like the "Darkness" and wait for specific events, such as a stalactite falling to bridge a gap. Eventually, you reach a well where a young child lowers a bucket.
This ending requires you to abandon your duty and find a way out of the subterranean kingdom. THE LONGING - ALL ENDINGS (400 Days, Good, Bad, Secret)
If you attempt to climb out of the well but fail, or if you interact with the Blind Man and choose to "end the longing" prematurely by jumping, you trigger a variation of the death ending. You must navigate to the upper-most parts of the cave system