Hero Siege Unlock Nightmare

In Season 10, the practical answer is: finish the Normal campaign route through Act 9. Official notes say completing Normal opens the next progression stage while preserving Normal waypoints; current community walkthroughs identify the Act 9 Cthulhu clear as the Normal milestone that unlocks Nightmare.

There is no official level number found for the Nightmare unlock in the reviewed Season 10 notes. Treat level as a power check rather than a confirmed gate unless your current in-game UI says otherwise.

If Nightmare does not appear, re-check Act 8 Odin, Act 9 quest completion, Cthulhu clear state, your quest log, and whether you are reading a pre-Season 10 guide.

Season 10 rewires difficulty progression. The official patch notes say players only progress through the Acts once, on Normal, and that after Normal is completed the game moves into Nightmare while retaining Normal waypoints.

That makes older advice about unlocking Nightmare by clearing an older end boss, replaying every act on multiple difficulties, or following pre-Act-9 route assumptions risky.

The reviewed official Season 10 sources do not publish a separate Nightmare level requirement. Community walkthroughs frame Nightmare as a Normal-campaign completion unlock rather than a level-gated feature.

That does not mean level never matters in practice. If your character cannot clear Act 9, level and gear are still real power checks, but they should not be written as a confirmed hidden unlock number without in-game proof.