A Taste of Hell

Survival Hell unlocks around Chapter 2 through Hell's Arena — it's a roguelite gauntlet with fixed (non-random) layouts.

Pick up item upgrades during a run; they stay permanent once you clear that run, so a full clean run through the early stages banks upgrades for later.

Clear any 3 stages — if you're defeated, you lose that run's rewards, so don't be afraid to reset and retry.