Key Sigils are a brand new addition to the game ... That being said, you’ll be back here a lot in the future, so there really is no rush. The hidden island is a weird one. No matter what you ...

The Diablo 4 Nightmare Dungeons and Nightmare Sigils become available as mechanics once you've cleared the Cathedral of Light Capstone Dungeon. You should also switch to World Tier 3: Nightmare.