diff --git a/src/sanity.lua b/src/sanity.lua
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index 0000000..1007809
--- /dev/null
+++ b/src/sanity.lua
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+-- The player's grip on their own mind, tracked as a slow-moving meter
+-- alongside hp and mana rather than a temporary status.lua effect: nothing
+-- brews or casts sanity away in a single stroke, it only bleeds out from
+-- sustained exposure to dread (standing in the dark with a spent torch,
+-- corpses piling up nearby, undead in plain sight, a curse whose bite is
+-- already known, an infested floor's own creeping wrongness) and claws
+-- back just as slowly, one calm turn - or one potion of clarity - at a
+-- time. Falling tiers cost real attack and defense, exactly like
+-- encumbrance.lua's overloaded penalty, and the worst tier risks losing a
+-- turn outright to panic or giving the player's own position away with an
+-- involuntary scream, so the meter is never just flavour text on the
+-- status panel.
+
+local Factions = require("factions")
+local Curses = require("curses")
+local Atmosphere = require("atmosphere")
+local Stealth = require("stealth")
+
+local Sanity = {}
+
+Sanity.MAX = 100
+
+Sanity.COMPOSED = "composed"
+Sanity.SHAKEN = "shaken"
+Sanity.TERRIFIED = "terrified"
+Sanity.BROKEN = "broken"
+
+-- Tier ceilings: a value at or below TERRIFIED_MAX but above 0 reads as
+-- TERRIFIED, and so on upward. 0 itself is always BROKEN regardless of
+-- where these ceilings sit.
+Sanity.TERRIFIED_MAX = 30
+Sanity.SHAKEN_MAX = 60
+
+Sanity.DARKNESS_DREAD = 2
+Sanity.CORPSE_DREAD_PER = 1
+Sanity.CORPSE_DREAD_CAP = 4
+Sanity.UNDEAD_DREAD_PER = 2
+Sanity.UNDEAD_DREAD_CAP = 6
+Sanity.CURSE_DREAD = 2
+Sanity.WEATHER_DREAD = 2
+
+-- Passive drift back toward composure on any turn that accrued no dread
+-- at all - never enough on its own to outrun a floor still actively
+-- haunting the player, but reliable once they step clear of it.
+Sanity.CALM_RECOVERY = 1
+
+Sanity.PANIC_NUMERATOR = 1
+Sanity.PANIC_DENOMINATOR = 3
+
+Sanity.SCREAM_NUMERATOR = 1
+Sanity.SCREAM_DENOMINATOR = 4
+Sanity.SCREAM_RADIUS = 6
+
+local ATTACK_PENALTY = {
+  [Sanity.COMPOSED] = 0, [Sanity.SHAKEN] = -1, [Sanity.TERRIFIED] = -3, [Sanity.BROKEN] = -5,
+}
+local DEFENSE_PENALTY = {
+  [Sanity.COMPOSED] = 0, [Sanity.SHAKEN] = -1, [Sanity.TERRIFIED] = -2, [Sanity.BROKEN] = -4,
+}
+
+local function chebyshev(ax, ay, bx, by)
+  return math.max(math.abs(ax - bx), math.abs(ay - by))
+end
+
+-- A fresh entity - or a monster, which never accrues any dread of its own
+-- since nothing ever calls Sanity.tick for one - reads as fully composed.
+function Sanity.current(entity)
+  return entity.sanity or Sanity.MAX
+end
+
+function Sanity.tierOf(entity)
+  local value = Sanity.current(entity)
+  if value <= 0 then return Sanity.BROKEN end
+  if value <= Sanity.TERRIFIED_MAX then return Sanity.TERRIFIED end
+  if value <= Sanity.SHAKEN_MAX then return Sanity.SHAKEN end
+  return Sanity.COMPOSED
+end
+
+function Sanity.adjust(entity, delta)
+  local updated = Sanity.current(entity) + delta
+  if updated > Sanity.MAX then updated = Sanity.MAX end
+  if updated < 0 then updated = 0 end
+  entity.sanity = updated
+  return updated
+end
+
+function Sanity.attackPenalty(entity)
+  return ATTACK_PENALTY[Sanity.tierOf(entity)]
+end
+
+function Sanity.defensePenalty(entity)
+  return DEFENSE_PENALTY[Sanity.tierOf(entity)]
+end
+
+-- Each dread source below mirrors the vocabulary an existing module already
+-- uses (Atmosphere's weather keys, Factions' undead tag, Curses' reveal
+-- flag) rather than inventing a parallel one, and every source caps its own
+-- contribution so no single cause can spike the meter to zero in one turn.
+
+function Sanity.darknessDread(game)
+  if not game.level.dark then return 0 end
+  if game.player.light.fuel > 0 then return 0 end
+  return Sanity.DARKNESS_DREAD
+end
+
+function Sanity.corpseDread(game)
+  local count = 0
+  for _, corpse in ipairs(game.level.corpses) do
+    if chebyshev(corpse.x, corpse.y, game.player.x, game.player.y) <= 1 then
+      count = count + 1
+    end
+  end
+  return math.min(count * Sanity.CORPSE_DREAD_PER, Sanity.CORPSE_DREAD_CAP)
+end
+
+-- Only the undead currently visible weigh on the mind - one lurking
+-- somewhere unseen on the floor cannot, unlike a nearby corpse the player
+-- can smell without looking.
+function Sanity.undeadDread(game)
+  local count = 0
+  local map = game.level.map
+  for _, monster in ipairs(game.level.entities) do
+    if monster:isAlive() and monster.faction == Factions.UNDEAD
+      and game.visible[map:index(monster.x, monster.y)] then
+      count = count + 1
+    end
+  end
+  return math.min(count * Sanity.UNDEAD_DREAD_PER, Sanity.UNDEAD_DREAD_CAP)
+end
+
+-- A curse still hiding behind an ordinary appearance has not yet given its
+-- wearer anything to dread - only a revealed one gnaws at the mind.
+function Sanity.curseDread(entity)
+  for _, slot in ipairs(Curses.SLOTS) do
+    local item = entity[slot]
+    if Curses.isCursed(item) and Curses.isRevealed(item) then return Sanity.CURSE_DREAD end
+  end
+  return 0
+end
+
+function Sanity.weatherDread(weather)
+  if weather == Atmosphere.INFESTED then return Sanity.WEATHER_DREAD end
+  return 0
+end
+
+function Sanity.totalDread(game)
+  return Sanity.darknessDread(game)
+    + Sanity.corpseDread(game)
+    + Sanity.undeadDread(game)
+    + Sanity.curseDread(game.player)
+    + Sanity.weatherDread(game.level.weather)
+end
+
+-- Advances the player's sanity by one turn: every dread source at once, or
+-- - on a turn none of them fired at all - a small step back toward
+-- composure instead. Returns a log line the instant the player's tier
+-- actually changes, in either direction, or nil while it holds.
+function Sanity.tick(game)
+  local before = Sanity.tierOf(game.player)
+  local dread = Sanity.totalDread(game)
+  local worsening = dread > 0
+  if worsening then
+    Sanity.adjust(game.player, -dread)
+  else
+    Sanity.adjust(game.player, Sanity.CALM_RECOVERY)
+  end
+  local after = Sanity.tierOf(game.player)
+  if after == before then return nil end
+  if worsening then
+    if after == Sanity.BROKEN then return "your mind gives way to raw terror" end
+    if after == Sanity.TERRIFIED then return "dread claws at the edges of your sanity" end
+    return "your nerves begin to fray"
+  end
+  if after == Sanity.COMPOSED then return "your mind is your own again" end
+  return "your nerves start to steady"
+end
+
+-- BROKEN alone risks losing the turn outright to a fit of terror; every
+-- other tier always lets the player act. Rolls against `rng` so a seeded
+-- run always panics on the identical turns.
+function Sanity.rollPanic(rng, entity)
+  if Sanity.tierOf(entity) ~= Sanity.BROKEN then return false end
+  return rng:chance(Sanity.PANIC_NUMERATOR, Sanity.PANIC_DENOMINATOR)
+end
+
+-- TERRIFIED or worse risks an involuntary scream that alerts every living
+-- monster within earshot (reuses stealth.lua's own wake path) - the fear
+-- itself becomes a hazard to whoever else is on the floor.
+function Sanity.rollScream(rng, game)
+  local tier = Sanity.tierOf(game.player)
+  if tier ~= Sanity.TERRIFIED and tier ~= Sanity.BROKEN then return false end
+  if not rng:chance(Sanity.SCREAM_NUMERATOR, Sanity.SCREAM_DENOMINATOR) then return false end
+  for _, monster in ipairs(game.level.entities) do
+    if monster:isAlive() and chebyshev(monster.x, monster.y, game.player.x, game.player.y) <= Sanity.SCREAM_RADIUS then
+      Stealth.alert(monster, game.player.x, game.player.y)
+    end
+  end
+  return true
+end
+
+return Sanity