Development Hell is a secret area/easter egg within Act I during the quest "A Shattered Crown".
When you go into one of the areas named "Defiled Crypt" (which is required for a quest goal) there is a chance you will instead be taken into Development Hell.
When you're in Development Hell, you'll see that normal Risen Dead are named after members of the Diablo III Development team! Jay Wilson, the game's director is an elite monster and killing him will unlock the Smash! Jay, Smash! secret Achievement as well as a Banner Sigil.
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