There is more than one meaning to Hell in the Diablo games. If you were linked here by an article pointing to one of these meanings, you should edit it to point directly to the intended meaning.
Hell in Diablo games can mean one of the following:
Location: Hell
The Burning Hells is the birthplace of demons like three Prime Evils of Diablo, Mephisto or Baal. They are in conflict with the High Heavens, where Angels like Tyrael rule.
Game Difficulty: Hell
The Hell difficulty is the hardest difficulty setting in the Diablo games. The three difficulties are: