Hell is the highest standard difficulty setting in Diablo II.
Diablo II organizes its campaign difficulties as Normal, Nightmare, and Hell. Players generally begin in Normal, unlock Nightmare after completing the campaign, and then progress to Hell. Hell greatly increases monster strength and introduces severe resistance penalties for player characters, making equipment, skill choices, and combat positioning much more important.
Nightmare is the middle difficulty, so it is a common mistaken answer. Inferno and Torment belong to later Diablo games or their systems rather than the original Diablo II difficulty sequence. Diablo II: Lord of Destruction retained the same three difficulty names while expanding the campaign with Act V.
Hell difficulty became one of the defining challenges of Diablo II. It encouraged repeated boss runs, careful resistance management, powerful item hunting, and cooperation between players. The name also fits the game’s setting, but here it specifically identifies the final standard difficulty tier.