UPDATE 6/28: This trick was addressed in Patch 1.0.3, which was released on June 27. That said, there is a new method that players can use to reset their Nightmare Dungeons, and it is detailed in this Diablo 4 Patch 1.0.3 XP farming guide.
Original story follows.
Nightmare Dungeons play a big role in Diablo 4's endgame, and some of them are great sources for both XP and loot. That said, the Sigils required to access these maps have made them difficult to efficiently farm, and many players have opted to grind in standard dungeons instead. However, it is actually possible to instantly and infinitely reset Nightmare Dungeons in Diablo 4, and this guide is here to provide details on how that is done.
To note, the Nightmare Dungeon reset strategy detailed below only works when playing solo. While it is possible that a revision will be discovered that allows groups to perform a similar trick, players that like to grind with their friends will need to use some more traditional XP farms in Diablo 4 for the time being.
Diablo 4: How to Reset Nightmare Dungeons
The first step toward endlessly farming a Nightmare Dungeon in Diablo 4 is obtaining a Nightmare Sigil for the dungeon of interest. Indeed, while this reset strategy can be used with any Nightmare Dungeon, targeting the best maps will undoubtedly increase the rate of XP acquisition. For those players that are curious, Aldurwood, Blind Burrows, and Champion's Demise are currently considered some of the best Nightmare Dungeons, though they are not the only good options.
Furthermore, fans that want to maximize their XP gains should make sure that the Nightmare Sigil's Tier is high enough so that the monsters inside the dungeon are three levels (or more) higher than their character. This is important because the maximum XP bonus is awarded for defeating enemies that are three levels more powerful than the player character. To determine the Sigil Tier that fans need to use to cause that to occur, they should simply subtract 50 from their character's level in Diablo 4.
Once a suitable Nightmare Sigil has been obtained, players should use it from within the Consumables section of their Inventories, enter the Nightmare Dungeon, and complete its first objective. When that is done, Diablo 4 players should leave the Nightmare Dungeon and make their way to any standard dungeon that they want. Fans should now complete the first objective in this standard dungeon and then leave it.
Players have now done all the preparatory work that is required to instantly reset Nightmare Dungeons, and they should return to the Nightmare Dungeon that they were in previously. Upon entering this map, fans will find that it has fully reset, and they can now perform resets any time that they want simply by selecting Leave Dungeon from the emote wheel or teleporting to a town. Notably, players should make sure that they do not fully complete the dungeon until they are ready to stop farming, as they will not be able to reset once they reach the end.
Diablo 4 is available for PC, PS4, PS5, Xbox One, and Xbox Series X/S.