From the publisher's web site:
The Secret Cow Level is the most popular Easter egg in computer gaming history, and now you can milk it for all it's worth in your D&D/Diablo games. All you need is a party of adventurers powerful enough to kill Diablo. You do have one of those, right? No? Well, send 'em back to Hell till they're tough enough to take out a demon lord. Because when they're done with that, a thousand moo-licious bovines intend to inflict udder depravity until they lay the adventurers low. Are you still checking your vision on that last paragraph? That's right, we said a thousand cows, all with poleaxes. Got damage resistance?