Theres a second secret level aside from whimsyshire, its called Development Hell.
Its a fourth crypt in the cemetery of the forsaken in act 1.
It only spawns on nightmare and above, so make an act 1 game on the quest A Shattered Crown, on nightmare or above.
And run to the cemetery of the forsaken.
Normally only 3 defiled crypts spawn but if your lucky there will be a fourth called Development Hell, if its not there just logout and resume game until it spawns.
Its a crypt full of zombies/skeletons all named after the games designers etc, and a rare named Jay Wilson.
http://postimage.org/image/cx2f2ghc7/
http://postimage.org/image/6sh5m71hx/