Lugaru Level 15
Also known as the Secret Cow Level. In order to travel to this level, you must have Wirt's wooden leg and a Tome of Town Portal. Put them in your Horadric Cube and transmute to open a portal in the Rogue Encampment.
Once you arrive, prepare yourself for a bodacious Bunny vs. Bossie battle. Good luck, but don't worry too much: if you've managed to actually get to Level 15 in Lugaru, you're a mighty unusual individual, and a few thousand halberd-wielding bovines shouldn't faze you.