It appears this level only appears in Nightmare, Hell and Inferno. Easy way to tell it spawned is when there's 4 tombs accessible in the graveyard in Act 1.
Inside there's 1 elite boss, and multiple zombies with names of game creators and their job titles.
Inside there's 1 elite boss, and multiple zombies with names of game creators and their job titles.
Nice Easter Egg.
http://www.youtube.com/watch?v=OzS2Z4z11vY