Development Hell is a new way of showing the credits, it is a rarely spawned level that features a group of zombies with the names of the Diablo III game developers.
"Development Hell" is a reference to Development hell, an abnormally long period of development for computer games or other various forms of electronic media.
Development Hell is found in the Cemetery of the Forsaken in Act I of Diablo III, as one of the Defiled Crypts. It will appear as a fourth accessible passage, as opposed to normal three, and will have the Development Hell name instead of Defiled Crypt.
The chance for it to appear is very low (roughly 1%).
This crypt features a variety of zombies: Ravenous Dead, Retching Cadavers, Hungry Corpses. All monsters have the names and jobs of the game developing group. Two unique monsters are present in this dungeon: Jay Wilson and Josh Mosqueira.
Killing Jay Wilson grants a hidden achievement; "Smash, Jay, Smash", placed in "Feat of Strength" group on the achievement page, plus a sigil of a horned demon.