The Mysterious Negg Cave is a secret daily in Shenkuu that is only accessible via clicking the mountain to the left of Remarkable Restoratives.
How to Solve[]
Within the cave, you're shown about two to six clues on a piece of parchment next to the Clockwork Negg, showing where each color and element should be placed on the grid and next to each other. First, you must overlay the colors with the elements. Second, remember that the colors red and yellow and the elements fire and water cannot overlap, and vice versa.
The Neggbreaker[]
The Neggbreaker is a giant Mutant Hasee that hunts Neggs. Once you solve the cave's puzzle, The Neggbreaker can jump out at you*, unlocking him as a Battledome challenger. He was introduced at the end of the 2012 Festival of Neggs, and a reward from the event, the Ancient Negg Amulet, was used to weaken his power.
*The Neggbreaker is currently unavailable in the new Battledome.