Diablo – reflected in the Mirrorball

It's a random event that has a chance of occurring in any game you make

I ran into it once and also failed. I'm fairly sure you have to either be pretty lucky in going the right direction, or flat out run through the place, ignoring the mobs.

We ran through the entire thing, but paused for a few seconds here or there to kill mobs when we accumulated to many. After a few dead ends we found the final room with the teleporter in it, but ran out of time right when we entered the it.

The only condition or requirement is that you must have completed the game on normal difficulty and be playing on at least nightmare, other than that, it is simply chance. If you keep reloading the area(by exiting and reentering the game), you should eventually come across it.

Please note that given the lack of information on these Easter Egg dungeons, the requirements above may not be entirely accurate, but thus far I have yet to see this dungeon, in game or on the internet, before nightmare difficulty.