In Diablo 4, Nightmare Dungeons are a higher-tier dungeon type that becomes available once you reach level 50. To access Nightmare Dungeons, you must first complete the campaign and reach level 50, after which you can use Nightmare Sigils to enter these challenging dungeons. The difficulty of Nightmare Dungeons scales with the level of the sigil used, and they offer more challenging enemies and better rewards compared to standard dungeons.