
Once you get a sigil, use it, and it’ll open up a dungeon for you. I recommend finishing dungeons that reward Grim Favors, so you end up with a bit of extra loot. These sigils are found in Grim Favor rewards from the Tree of Whispers. To access nightmare dungeons, you’ll need to get a sigil first.

When you start tier 3 difficulty, you’ll be introduced to nightmare dungeons and Helltide. Once you have finished the capstone dungeon, you can freely change the difficulty to tier 3 now. However, you can clear the capstone dungeon with the right gear and a good leveling build as early as 45.

The dungeon will scale to a minimum of level 50 monsters, so if you are lower than that level, they won’t scale down any lower. It requires you to finish the Cathedral of Light capstone dungeon. However, if you finish the campaign fast enough, you’ll see the tier 3 world difficulty quest under your priority quests. Since nightmare difficulty sets the monsters to level 50 and above, you should unlock it around that player level.

To unlock tier 3 or nightmare difficulty, you’ll need to finish the campaign first. Things to Know Before Using the Sigil Unlocking Tier 3 or Nightmare Difficulty
