Uber Monsters are special quest monsters added in Diablo 2 patches v1.10 and v1.11. They can only be battled on the Battle.net Realms, and are difficult to find. They are also extremely difficult, with far higher stats and new abilities above and beyond those found on normal monsters in the game. The rewards they drop are far better as well, of course.
These Uber monsters and events have also been carried over to Diablo 2 Resurrected, and once again will be online only. These events are not available over LAN or single-player.
You’ll encounter all but one of them as part of the Pandemonium Event which takes place in new versions of game areas including Matron’s Den, Forgotten Sands, Furnace of Pain and Uber Tristram.
The Uber Diablo event takes place where the next SuperUnique to spawn. Players need to find him.