When you first start Diablo 4 you’ll only have access to two World Tiers (difficulty modes) - World Tier I (Adventurer) and World Tier II (Veteran). These two World Tiers are functionally indistinct. Enemies will be stronger in World Tier II and you’ll get somewhat better monetary and XP rewards, but the loot pools are the same and whether these gains outpace the decrease in kill rate is dubious, at best. World Tier III (Nightmare) is another matter entirely, significantly boosting XP gains and giving the player access to new tiers of loot, events, and dungeons. Unlocking Nightmare difficulty isn’t as straight-forward as visiting the World Tier Statue in 
Kyovashad, however - you’ll have to prove your competence to unlock it. This page will cover what changes in Nightmare difficulty and how to unlock Nightmare difficulty.
(1 of 2) Nightmare difficulty unlocks Helltide events and Nightmare Dungeons,
Nightmare difficulty unlocks Helltide events and Nightmare Dungeons, (left), and allows you to find higher-tier gear, like Sacred items and Uniques. (right)
What is World Tier 3 in Diablo 4?¶
There are four World Tiers in Diablo 4, the first two of which are analogous to normal difficulty in Diablo 2 (Veteran is a bit more of a challenge than Adventurer, but aside from inflated enemy stats it’s not fundamentally different from Adventurer). World Tier 3, dubbed Nightmare difficulty, is fittingly similar to Nightmare difficulty in Diablo 2. Your resistances will be lower, enemies are appreciably more powerful, and drop higher-tier loot which cannot be found in lower difficulties. The changes found in Nightmare difficulty are as follows:
- Sacred and Unique items can drop.
- Nightmare Sigils can drop that unseal Nightmare Dungeons.
- Helltides can appear across Sanctuary.
- Champion monsters with damage resistance auras can appear.
- Monsters give 100% increased Experience.
- Monsters drop 15% more Gold.
- Monsters overcome 20% Resistance.
Sacred items have higher stats and scale to the player’s level better than lower-quality items, while Unique items are a rarity tier above even Legendary gear. The two of them conspire to make your gear from Adventurer/Veteran difficulties redundant, or optimistically, this new gear exists to give you the strength you need to springboard into World Tier IV. Both Sacred and Unique items can drop anywhere, anytime, as long as the game is set to World Tier III, but Nightmare Dungeons and Helltide events give you new gameplay loops to grind, if you’re looking for some variety. Enemies will be appreciably more difficult in Nightmare difficulty however, in no small part due to Champion spawns and a flat negation of 20% of your damage reduction.
Better loot, new gameplay elements, harder enemies… everything you could want in Nightmare difficulty, really.
(1 of 2) To unlock World Tier 3, you must complete the Cathedral of Light Capstone Dungeon,
To unlock World Tier 3, you must complete the Cathedral of Light Capstone Dungeon, (left), after which you can visit the World Tier Statue and switch to Nightmare difficulty at any time. (right)
How to Unlock World Tier 3 in Diablo 4¶
Unlike Adventurer and Veteran difficulty, you cannot switch to Nightmare difficulty at any time. The first precondition is completing the main questline with at least one character (subsequent characters can skip the campaign), after which you still must prove that you’re powerful enough to contend with the legions of hell in Nightmare difficulty. To do this you must complete the priority quest World Tier 3: Nightmare, which will take you to the Cathedral of Light Capstone Dungeon. Complete this dungeon on World Tier 2 (Veteran difficulty) and you’ll unlock World Tier 3.
The 
Cathedral of Light Capstone Dungeon is geared towards Lv50 characters, but it should be noted that the gear you can farm (especially Legendary gear whose legendary modifiers synergize with your build) on World Tier 1 can prove more than sufficient to clear this dungeon with some ease, and even contend with the enemies on World Tier 3, making World Tier 2 something of an extraneous difficulty.
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