I stumbled upon this random dungeon a few days ago during a quest. I started going through it, but I didn't complete it (computer problems). What's in the dungeon and how deep does it go? Is there a way I can get back to it so I can explore it again?
It is a random spawn in the Weeping Hollow zone. Whenever Diablo 3 generates a zone it populates it with the appropriate scenery plus a certain number of events and/or dungeons. The Den of the Fallen is one of these - a two level dungeon, generally with a selection of rare monsters, champion packs or Resplendent Chests on the second level.
To return to it, you will need to explore the Weeping Hollow repeatedly until it spawns again.