Simple. When you are 14 you can go there. Go through the door in the garden room when you are 14+ or when you are 16+ it will show on the map.
Good Luck.
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To get to the chaos crypt you will have to go to SHADOWFALL and talk to GRAVELYN and do the CHAOS CRYPT quest.
There are not secret levels in this game.
You have to get the secret exit's in every world and the secret worlds
There are secret exits in levels look it up on youtube on the secret exits.
Well my guess would be Arceus.