The World Event was a feature in Diablo II that was added in Patch 1.10.
The World Event occurs randomly, and is unaffected by anything performed in the game. Many players received it while just sitting in a game, not killing anything or selling any Stones of Jordan. World Events occurred in all difficulties, as well as in Hardcore, Ladder, and normal Realm games. We've had far more reports of them in Hell games than Normal or Nightmare, but it occurs in those games as well.
When the World Event triggers, orange text proclaiming, "Diablo Walks The Earth" would appear. The sky will go black and a circle of light would surround the player.
Once the message appears, the next super-unique monster that spawns would be Über Diablo (aka Diablo Clone). It was best to run right to a location that always had a super-unique in it, such as Eldritch, north of the first waypoint in Act 5. Once Diablo had been defeated, he may drop a Annihilus charm. Many players fought Über Diablo in place of Rakanishu, Pindleskin, The Countess and Baal's Minions.