Hard Mode
Hard Mode is an optional tougher take on the Ruins, available from a fresh file when you name the fallen human a specific way (the game tells you when it applies). Expect denser encounters and less forgiveness while you re-learn early patterns.
Hard Mode is a challenge layer, not a full alternate campaign through the entire Underground. Use it after you already understand FIGHT / ACT / MERCY.
Related: Combat basics · Ruins · Endings hub.