Diablo Clone and its reward Annihilus is event that only exist in the Battle.net servers of the game. Unlike Pandemonium event, this event is random and you cant do it whenever you feel like it. The clone (Uber Diablo) can appear in any Hell difficulty game regardless of an open game or password game. If you manage to kill him, he drops a unique Small Charm - or "Anni" for short.
How to know where he might spawn?
He spawns by people selling Stone of Jordan unique rings to vendors. Yes .... you are not halucinating.... people are vendoring SoJs to make him spawn. You will notice "6433 Stones of Jordan sold to Merchants" appear as a message in your game. This means that someone is selling SoJs on the server you are connected so there is a high chance that he might spawn inside your game so do not exit the game! Dont be shocked by the number of sold SoJs that appear in the message. These are the total SoJs sold on this server from the start of Battle.net. If he indeed spawns in your game you will see a message "Diablo Walks the Earth".
Back in the day there used to be a way of knowing which server you are connected to and try do make as many games with your frinds as possible ending up on the same server on which the SoJs are being sold.
The way to do it is while in the game where you see that SoJs are being sold, alt tab and open a Windows Command Prompt (CMD). Type netstat -n | find "4000" and look for the IP address that will come up with a connection on port 4000 and status "ESTABLISHED" connection (screenshot example below). So we have identified the IP address of the server on which the SoJs are being sold. Ask friends to join games and do the same with the command prompt window until they hit the same server IP address as you in their games. If they manage to do it, just wait if he spawns.
Ok I was lucky and he spawned in my game but where to find him?
A lot of places will tell you that he can spawn in all Acts in a place of a superunique monster but most of the times you will find him on Rakanishu's place in Stony Field Act 1 or Bishibosh in Cold Plains Act 1. First go there and if you don't find him, start going to superunique mobs locations.
Is he strong? Can you solo him?
He is different from Pandemonium Diablo! Seems a little weaker from stats standpoint. He casts Armageddon and some other non-Diablo abilities + all of his nomral abilities. He also regenerates health at 1.2% per sec rate. Just like the other Ubers