Nightmare Dungeons in Diablo 4 offer a challenging but rewarding experience, ideal for endgame progression. These dungeons require a Nightmare Sigil to access and feature higher monster levels and density.

Nightmare Dungeons drop valuable loot, including Glyphs, Legendaries, and Uniques. Farming Nightmare Dungeons is the best way to acquire additional Sigils and advance your character. Here are steps to unlock Nightmare Dungeons.

1. Finish the Main Storyline
2. Complete the Capstone Dungeon on World Tier 2
3. Change Game Difficulty to World Tier 3
4. Complete Whispers at Tree of Whispers
5. Activate the Nightmare Sigil

1. Finish the Main Storyline

Completing the Diablo 4 main campaign in either World Tier 1 or World Tier 2 is the first crucial step in your journey to unlocking Nightmare Dungeons. This involves progressing through all the acts and defeating Lilith. By the end of the main storyline, you'll have gained valuable experience, loot, and skills that will prepare you for the challenges that lie ahead in the Nightmare Dungeons.

2. Complete the Capstone Dungeon on World Tier 2

Once you've conquered the main campaign, your next task is to complete the Capstone Dungeon in World Tier 2. To gain access to Capstone Dungeon, you need to be at least level 50 and head to the Cathedral of Light, which is located north of Kyovashad. Successfully beating Capstone Dungeon will automatically open up World Tier III and unlock Nightmare Dungeons, which need Nightmare Sigils to enter.

3. Change Game Difficulty to World Tier 3

After completing the Capstone Dungeon, you'll have the option to change your game difficulty to World Tier 3. There are 2 ways to change World Tier. You can either change it in the character select screen or change it using World Tier Status in the main cities like Kyovashad.

This increases the difficulty level but also offers better rewards and loot. Switching to World Tier 3 is required to unlock Nightmare Dungeons, as it sets the stage for the high-level challenges and rewards that these special dungeons offer.

4. Complete Whispers at Tree of Whispers

To get your first Nightmare Sigil in Diablo 4, complete 10 Whispers in World Tier 3 and submit them to the Tree of Whispers to receive a reward cache. Open the cache in your inventory to find a Tier 1 Nightmare Sigil among the rewards.

After getting your first Nightmare Sigil, they can be additionally achieved through various methods in the future.

1. Completing Whispers of the Dead: Engage in endgame activities with the Tree of Whispers to earn Grim Favors. Accumulate enough to claim Nightmare Sigils as rewards, unlocking challenging Nightmare Dungeons.
2. Nightmare Dungeon Runs: Once you reach level 50 and enter the Nightmare World Tier, Nightmare Sigils can drop randomly in dungeons. These sigils activate Nightmare Dungeons, offering high-quality loot and glyph upgrades.
3. Completing Events: Participate in bounty and seasonal events after reaching World Tier 3 - Nightmare. Completing these events can yield Nightmare Sigils, which are essential for accessing Nightmare Dungeons.
4. Defeating World Bosses: Take on high-level World Bosses scattered across the map. Defeating them not only grants high-quality loot but also a chance to obtain Nightmare Sigils for upgrading dungeons.
5. Crafting at the Occultist: Complete a Tier 3 Nightmare Dungeon to unlock the crafting ability at the Occultist. Use Sigil Powder, obtained by salvaging lower-tier Nightmare Sigils, to craft higher-tier ones for advanced gameplay.

5. Activate the Nightmare Sigil

Once you've obtained a Nightmare Sigil, you'll need to activate the sigil from your inventory. This will transform a regular dungeon into a Nightmare Dungeon, significantly ramping up the difficulty but also the rewards.

To activate Nightmare Sigil, follow these steps:

1. Navigate to your Consumables Tab and locate the Nightmare Sigil.
2. Click on "Accept" when the "Consume Sigil" pop-up appears.
3. Your first Nightmare Dungeon is now activated.

After you activate your first Nightmare Dungeon, you can either travel there or simply open the map and it will ask you if you want to be teleported there.