When enough Stone Ring of Jorden rings have been sold to venders in Hell mode, the Uber Diablo Clone will appear. In D2 LoD he will only appear for games on the same server. In D2 Resurrected he will appear for all games in the region. The current stage will be show in the console. Once it hits stage 6, he will appear. He will replace any unique boss that has not yet spawned in that game. Common places spawn/fight him are at the Carren Stones in The Stony Field, or Shenk at the top of The Bloody Foothills. When he dies, he will drop an Annihilus charm, which is the only unique charm in the game and well worth obtaining for any character build.
You can check the current stage by typing /uberdiablo
STAGES
1) Terror gazes upon Sanctuary
2) Terror approaches Sanctuary
3) Terror begins to form within Sanctuary
4) Terror spreads across Sanctuary
5) Terror is about to be unleashed upon Sanctuary
6) Diablo has invaded Sanctuary