To unlock the secret level, you will need to collect all 10 hidden eggs. You can return to past chapters to find any eggs you missed.
Once you have all 10 eggs, start Chapter 26 and go down the ladder to your left. Then, head to your right into the darkness until you can’t go any further, and then jump to keep going right. You will reach a secret door. If you’ve collected all 10 eggs, the door will open and give you access to a very challenging secret level.