Ascended Fanon: Wild rumors spread, for no apparent reason, of a Secret Cow Level in Diablo. There wasn't one. Blizzard, taking it all in good fun, made "thereisnocowlevel" a cheat code in Starcraft... and then put a Secret Cow Level in Diablo II.
The Other Darrin: Subverted. The voice actor for Baal is credited in Diablo II Classic as Milton James and in Lord of Destruction as Richard Barnes, but they are in fact the same person.
Refitted for Sequel: If you scroll through the unused dialogue from the previous game, you might notice a few familiar names — Izual, Andariel, and Horazon were all meant to appear in the first game, before being cut.
In the Battle.net chat interface in Diablo II, there is a gem that can be clicked on to toggle a lit/unlit state, but appears to do nothing functional. Occasionally when you click on it, it will give a message, such as "Perfect gem activated." Hundreds of rumors about the gem's actual, secret function have spawned. All Blizzard has said about it is that it is "Working as intended." It is possible, by clicking the gem a certain number of times, to trigger a message saying "Perfect Game Activated" (normally the message is "Gem Activated/Deactivated"), but the number of clicks required to do so is random and has no effect on anything else.
Another long-enduring Diablo II legend is Reziarfg, a monster named for one of Diablo II's developers, created by Blizzard's Battle.net staff at the Arreat Summit as an April Fools joke, with stats and some official-sounding lore to back it up. Many Hardcore characters have lost their lives trying to find the elusive beast.