Diablo IV launched with four World Tiers.
World Tiers are selectable difficulty settings that change the challenge and rewards of Diablo IV’s shared world. At launch, the four tiers were Adventurer, Veteran, Nightmare, and Torment. Players normally begin in the first two tiers and unlock higher tiers by completing specific Capstone Dungeons.
Higher World Tiers introduce stronger enemies and improve the quality or availability of rewards. Nightmare and Torment also connect to later progression systems, including Nightmare Dungeons and more powerful item categories. The system gives players a structured way to increase difficulty without simply choosing a single campaign setting.
World Tiers are not the same as character levels, seasonal ranks, or the number of difficulty options used in earlier Diablo games. Later updates changed Diablo IV’s progression and difficulty structure, so “at launch” is important: the answer refers to the game’s original 2023 configuration, not every later version.