Diablo II Easter Eggs
If a user is to type in 'soundchaosdebug' in the chat window on Battle.net or in single player mode, every sound file in the game is played in a chaotic muddle which lasts until the player reenters the code.
Secret Cow Level
The "Secret Cow Level" is the result of a running joke from the original Diablo that spawned from an Internet rumor about a cow which appears in the game, seemingly without purpose. Supposedly, if the cow was clicked on a certain number of times, a portal to a secret level would open. The rumor was a hoax, but the legend was born, and player after player asked Blizzard about how to access the level. In Diablo: Hellfire, the only expansion to the original Diablo (This "expansion" is actually considered an "add-on", since it was not an official release, but instead released by Sierra Entertainment), it was possible to change a parameter in a specific text file (Command.txt), so that the farmer who gives out the "rune bomb" quest was dressed in a cow suit, with appropriate new dialogue ("Moo." "I said Moo!"). This added fuel to the fire. To kill the rumor, Blizzard included a cheat (that automatically won the game) in StarCraft that read "There is no cow level", this being Blizzard's way of officially confirming that there was, in fact, no Cow Level. Among online game enthusiasts, this phrase has become an Internet joke similar to the phrase There Is No Cabal. On April 1, 1999, a Diablo II Screenshot of the Week featured cows fighting. People wondered if the screenshot was an April Fool's joke or if there really was a Secret Cow Level planned for Diablo II. It turned out that there was a cow level. To access the level, one must kill Diablo (or, in Lord of Destruction, kill Baal), return to Rogue Encampment in Act I within the same difficulty level, and then transmute Wirt's Leg with a Tome of Town Portal in the Horadric Cube. This will open a portal to the secret level (defeating Baal in the difficulty that the player wishes to enter the cow level in is no longer required, as of patch 1.11b. One only needs to be able to access a certain difficulty to enter its cow level). There is an item set named the "Cow King's Leathers" which may only be collected in this secret level. However, when a player kills the Cow King for their first time on a difficulty level, the character used to open the portal becomes unable to reopen the portal to the level on that difficulty level.
There exists a purple chat gem that is only visible when inside of a chat channel. There are many theories about the purpose of this gem. There is, however, no purpose for this gem aside from a meaningless message. The gem toggles between an active and inactive state. When activated, a chat message appears stating, "Gem Activated" and likewise, a message "Gem Deactivated" is shown when it becomes inactive. It can also say "Perfect Gem Activated" but the chances however are very low. It has also been said that it sometimes says "mooooooo"