diff --git a/internal/plugin/dnd_expedition.go b/internal/plugin/dnd_expedition.go
index 227dfd3..53676cb 100644
--- a/internal/plugin/dnd_expedition.go
+++ b/internal/plugin/dnd_expedition.go
@@ -364,6 +364,16 @@ func applyThreatDelta(expID string, delta int, reason string) error {
 	return err
 }
 
+// updateTemporalStack persists the zone-temporal stack (heat / instability).
+func updateTemporalStack(expID string, stack int) error {
+	_, err := db.Get().Exec(`
+		UPDATE dnd_expedition
+		   SET temporal_stack = ?,
+		       last_activity = CURRENT_TIMESTAMP
+		 WHERE expedition_id = ?`, stack, expID)
+	return err
+}
+
 // advanceExpeditionDay bumps current_day by one. Real-time day cycle (E1d)
 // calls this from the 06:00 cron.
 func advanceExpeditionDay(expID string) error {
diff --git a/internal/plugin/dnd_expedition_camp.go b/internal/plugin/dnd_expedition_camp.go
index be478f8..010b7f0 100644
--- a/internal/plugin/dnd_expedition_camp.go
+++ b/internal/plugin/dnd_expedition_camp.go
@@ -241,6 +241,14 @@ func processOvernightCamp(e *Expedition) string {
 		_ = applyThreatDelta(e.ID, -5, "long rest in fortified camp")
 	}
 
+	// §7.3 Underforge: fortified rest drops heat stacks by 2.
+	heatReduced := 0
+	if (kind == CampTypeFortified || kind == CampTypeBase) && e.ZoneID == ZoneUnderforge {
+		before := e.TemporalStack
+		after := reduceUnderforgeHeat(e)
+		heatReduced = before - after
+	}
+
 	// Auto-break the camp now that the rest has been applied.
 	_ = updateCamp(e.ID, nil)
 	e.Camp = nil
@@ -255,8 +263,12 @@ func processOvernightCamp(e *Expedition) string {
 	case CampTypeStandard:
 		return fmt.Sprintf("Long rest: HP %d → %d, spell slots & resources refreshed.", prevHP, c.HPCurrent)
 	case CampTypeFortified, CampTypeBase:
-		return fmt.Sprintf("Fortified rest: HP %d → %d (+1d6 = %d bonus); threat clock −5; resources refreshed.",
+		summary := fmt.Sprintf("Fortified rest: HP %d → %d (+1d6 = %d bonus); threat clock −5; resources refreshed.",
 			prevHP, c.HPCurrent, bonusHP)
+		if heatReduced > 0 {
+			summary += fmt.Sprintf(" Heat stacks −%d.", heatReduced)
+		}
+		return summary
 	}
 	return ""
 }
diff --git a/internal/plugin/dnd_expedition_cycle.go b/internal/plugin/dnd_expedition_cycle.go
index 56fc3b2..3231cee 100644
--- a/internal/plugin/dnd_expedition_cycle.go
+++ b/internal/plugin/dnd_expedition_cycle.go
@@ -157,7 +157,7 @@ func (p *AdventurePlugin) deliverBriefing(e *Expedition, now time.Time) error {
 	forceDouble := applyZoneTemporalPreBurn(e, e.CurrentDay+1)
 
 	// Advance day + supply burn happen together at the morning rollover.
-	harsh := e.ThreatLevel > 60 || e.TemporalStack > 0
+	harsh := e.ThreatLevel > 60 || zoneTemporalHarsh(e)
 	newSupplies, burn := applyDailyBurn(e.Supplies, harsh, e.SiegeMode || forceDouble)
 	if err := updateSupplies(e.ID, newSupplies); err != nil {
 		return err
diff --git a/internal/plugin/dnd_expedition_temporal.go b/internal/plugin/dnd_expedition_temporal.go
index 38285ae..97a7eed 100644
--- a/internal/plugin/dnd_expedition_temporal.go
+++ b/internal/plugin/dnd_expedition_temporal.go
@@ -32,6 +32,44 @@ type ZoneTemporalEffects struct {
 	// TidalActive — Sunken Temple Day 6 only. Combat: +1d6 cold per
 	// turn while wading; kuo-toa +2 AC, +1d4 attack rolls.
 	TidalActive bool
+
+	// UnderforgeHeatStacks — §7.3, 0–10. 1–3 ambient, 4–6 +1d4 fire
+	// per round in combat, 7–9 disadvantage on CON saves, 10 -2 to
+	// all rolls + forced rest/extraction.
+	UnderforgeHeatStacks int
+
+	// UnderforgeHeatBand classifies the stacks into the spec's
+	// behavior bands: "ambient", "warning", "supply", "critical".
+	UnderforgeHeatBand string
+}
+
+// UnderforgeHeatBandFor returns the §7.3 band label for a heat count.
+func UnderforgeHeatBandFor(stacks int) string {
+	switch {
+	case stacks <= 0:
+		return "none"
+	case stacks <= 3:
+		return "ambient"
+	case stacks <= 6:
+		return "warning"
+	case stacks <= 9:
+		return "supply"
+	default:
+		return "critical"
+	}
+}
+
+// zoneTemporalHarsh reports whether per-zone temporal state forces
+// harsh-conditions burn for THIS day's burn calculation. (Force-double
+// — overriding HarshMod with a 2× floor — uses the separate pre-burn
+// hook instead.)
+func zoneTemporalHarsh(e *Expedition) bool {
+	switch e.ZoneID {
+	case ZoneUnderforge:
+		// §7.3: heat 7–9 → supply burn +50%. Heat 10 also harsh.
+		return e.TemporalStack >= 7
+	}
+	return false
 }
 
 // applyZoneTemporalPreBurn runs at the top of deliverBriefing, before
@@ -62,10 +100,15 @@ func applyZoneTemporalPostRollover(e *Expedition) []string {
 		return sunkenTempleTemporalPostRollover(e)
 	case ZoneManorBlackspire:
 		return manorTemporalPostRollover(e)
+	case ZoneUnderforge:
+		return underforgeTemporalPostRollover(e)
 	}
 	return nil
 }
 
+// UnderforgeMaxHeat is the §7.3 cap.
+const UnderforgeMaxHeat = 10
+
 // ─────────────────────────────────────────────────────────────────────
 // §7.1 — Sunken Temple, Tidal Event (Day 6)
 // ─────────────────────────────────────────────────────────────────────
@@ -137,3 +180,78 @@ func manorTemporalPostRollover(e *Expedition) []string {
 		line)
 	return []string{line}
 }
+
+// ─────────────────────────────────────────────────────────────────────
+// §7.3 — The Underforge, Heat Accumulation
+// ─────────────────────────────────────────────────────────────────────
+
+// underforgeTemporalPostRollover increments Heat Stacks by 1/day (cap
+// 10), persists the change, and emits narration on band-crossing into
+// "warning" (4) and "critical" (10). Boss-defeated suppresses further
+// accumulation but does not reset existing heat — the pit stays hot.
+func underforgeTemporalPostRollover(e *Expedition) []string {
+	if e.BossDefeated {
+		return nil
+	}
+	prev := e.TemporalStack
+	if prev >= UnderforgeMaxHeat {
+		return nil
+	}
+	next := prev + 1
+	if next > UnderforgeMaxHeat {
+		next = UnderforgeMaxHeat
+	}
+	e.TemporalStack = next
+	if err := updateTemporalStack(e.ID, next); err != nil {
+		return nil
+	}
+	prevBand := UnderforgeHeatBandFor(prev)
+	newBand := UnderforgeHeatBandFor(next)
+
+	var lines []string
+	switch {
+	case prevBand != "warning" && newBand == "warning":
+		// First crossing into 4–6.
+		line := flavor.Pick(flavor.UnderforgHeapWarning)
+		line = strings.ReplaceAll(line, "[N]", fmt.Sprintf("%d", next))
+		_ = appendExpeditionLog(e.ID, e.CurrentDay, "temporal",
+			fmt.Sprintf("heat stack %d — warning band: +1d4 fire damage in combat rounds", next),
+			line)
+		lines = append(lines, line)
+	case prevBand != "supply" && newBand == "supply":
+		// Crossed into 7–9: supply +50%, dis on CON saves.
+		line := flavor.Pick(flavor.UnderforgHeapWarning)
+		line = strings.ReplaceAll(line, "[N]", fmt.Sprintf("%d", next))
+		_ = appendExpeditionLog(e.ID, e.CurrentDay, "temporal",
+			fmt.Sprintf("heat stack %d — supply burn +50%%, disadvantage on CON saves", next),
+			line)
+		lines = append(lines, line)
+	case prevBand != "critical" && newBand == "critical":
+		// Crossed into 10: exhaustion threshold.
+		line := flavor.Pick(flavor.UnderforgHeapCritical)
+		_ = appendExpeditionLog(e.ID, e.CurrentDay, "temporal",
+			"heat stack 10 — exhaustion: -2 to all rolls; rest in fortified camp or extract",
+			line)
+		lines = append(lines, line)
+	}
+	return lines
+}
+
+// reduceUnderforgeHeat is called by processOvernightCamp when the
+// active camp is fortified (or base) in the Underforge — long rest
+// drops heat stacks by 2 per §7.3. Returns the new stack count.
+func reduceUnderforgeHeat(e *Expedition) int {
+	if e.ZoneID != ZoneUnderforge {
+		return e.TemporalStack
+	}
+	next := e.TemporalStack - 2
+	if next < 0 {
+		next = 0
+	}
+	if next == e.TemporalStack {
+		return next
+	}
+	e.TemporalStack = next
+	_ = updateTemporalStack(e.ID, next)
+	return next
+}
diff --git a/internal/plugin/dnd_expedition_temporal_test.go b/internal/plugin/dnd_expedition_temporal_test.go
index 2a35ccb..8609427 100644
--- a/internal/plugin/dnd_expedition_temporal_test.go
+++ b/internal/plugin/dnd_expedition_temporal_test.go
@@ -247,6 +247,154 @@ func TestManorReset_PredicateOnly(t *testing.T) {
 	}
 }
 
+func TestUnderforge_HeatStackIncrementsAndBands(t *testing.T) {
+	setupZoneRunTestDB(t)
+	uid := id.UserID("@exp-uf-heat:example")
+	defer cleanupExpeditions(uid)
+
+	exp, err := startExpedition(uid, ZoneUnderforge, "",
+		ExpeditionSupplies{Current: 60, Max: 60, DailyBurn: 2, HarshMod: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	p := &AdventurePlugin{}
+
+	// Drive 11 mornings — heat should top out at 10.
+	for i := 0; i < 11; i++ {
+		fresh, _ := getExpedition(exp.ID)
+		if _, err := db.Get().Exec(
+			`UPDATE dnd_expedition SET last_briefing_at = NULL WHERE expedition_id = ?`,
+			exp.ID); err != nil {
+			t.Fatal(err)
+		}
+		// Top up supplies so we don't hit starvation; not relevant here.
+		if _, err := db.Get().Exec(
+			`UPDATE dnd_expedition SET supplies_json = ? WHERE expedition_id = ?`,
+			`{"current":99,"max":99,"daily_burn":2,"harsh_mod":2,"foraged_today":false,"packs_standard":3,"packs_deluxe":0}`,
+			exp.ID); err != nil {
+			t.Fatal(err)
+		}
+		fresh, _ = getExpedition(exp.ID)
+		if err := p.deliverBriefing(fresh, time.Now().UTC()); err != nil {
+			t.Fatalf("briefing %d: %v", i, err)
+		}
+	}
+	got, _ := getExpedition(exp.ID)
+	if got.TemporalStack != 10 {
+		t.Errorf("heat stack = %d, want 10 (capped)", got.TemporalStack)
+	}
+
+	// Verify warning + critical narrations both fired.
+	rows, _ := db.Get().Query(
+		`SELECT summary FROM dnd_expedition_log WHERE expedition_id = ? AND entry_type = 'temporal' ORDER BY entry_id`,
+		exp.ID)
+	defer rows.Close()
+	var summaries []string
+	for rows.Next() {
+		var s string
+		_ = rows.Scan(&s)
+		summaries = append(summaries, s)
+	}
+	want := []string{"warning band", "+50%", "exhaustion"}
+	for _, w := range want {
+		found := false
+		for _, s := range summaries {
+			if strings.Contains(s, w) {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("missing temporal entry containing %q in %v", w, summaries)
+		}
+	}
+}
+
+func TestUnderforge_HarshAtHeat7(t *testing.T) {
+	setupZoneRunTestDB(t)
+	uid := id.UserID("@exp-uf-harsh:example")
+	defer cleanupExpeditions(uid)
+
+	exp, err := startExpedition(uid, ZoneUnderforge, "",
+		ExpeditionSupplies{Current: 60, Max: 60, DailyBurn: 2, HarshMod: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Heat 6 — should NOT be harsh.
+	if _, err := db.Get().Exec(
+		`UPDATE dnd_expedition SET temporal_stack = 6, current_day = 7 WHERE expedition_id = ?`,
+		exp.ID); err != nil {
+		t.Fatal(err)
+	}
+	exp, _ = getExpedition(exp.ID)
+	if zoneTemporalHarsh(exp) {
+		t.Error("heat=6 should not trigger harsh")
+	}
+	// Heat 7 — should be harsh.
+	if _, err := db.Get().Exec(
+		`UPDATE dnd_expedition SET temporal_stack = 7 WHERE expedition_id = ?`,
+		exp.ID); err != nil {
+		t.Fatal(err)
+	}
+	exp, _ = getExpedition(exp.ID)
+	if !zoneTemporalHarsh(exp) {
+		t.Error("heat=7 should trigger harsh")
+	}
+}
+
+func TestUnderforgeHeatBandFor(t *testing.T) {
+	cases := []struct {
+		stacks int
+		want   string
+	}{
+		{0, "none"},
+		{1, "ambient"}, {3, "ambient"},
+		{4, "warning"}, {6, "warning"},
+		{7, "supply"}, {9, "supply"},
+		{10, "critical"},
+	}
+	for _, c := range cases {
+		if got := UnderforgeHeatBandFor(c.stacks); got != c.want {
+			t.Errorf("UnderforgeHeatBandFor(%d) = %q, want %q", c.stacks, got, c.want)
+		}
+	}
+}
+
+func TestUnderforge_FortifiedRestReducesHeat(t *testing.T) {
+	setupZoneRunTestDB(t)
+	uid := id.UserID("@exp-uf-rest:example")
+	defer cleanupExpeditions(uid)
+
+	exp := &Expedition{
+		ID:          "fake-uf",
+		ZoneID:      ZoneUnderforge,
+		TemporalStack: 5,
+	}
+	// Insert minimal row so updateTemporalStack works.
+	if _, err := db.Get().Exec(`
+		INSERT INTO dnd_expedition (expedition_id, user_id, zone_id, run_id, status, start_date, current_day, supplies_json, threat_events, region_state, last_activity, temporal_stack)
+		VALUES (?, ?, ?, NULL, 'active', CURRENT_TIMESTAMP, 1, '{}', '[]', '{}', CURRENT_TIMESTAMP, 5)`,
+		exp.ID, string(uid), string(ZoneUnderforge)); err != nil {
+		t.Fatal(err)
+	}
+	defer db.Get().Exec(`DELETE FROM dnd_expedition WHERE expedition_id = ?`, exp.ID)
+
+	got := reduceUnderforgeHeat(exp)
+	if got != 3 {
+		t.Errorf("reduceUnderforgeHeat(5) = %d, want 3", got)
+	}
+	exp.TemporalStack = 1
+	got = reduceUnderforgeHeat(exp)
+	if got != 0 {
+		t.Errorf("reduceUnderforgeHeat(1) = %d, want 0 (no underflow)", got)
+	}
+	// Wrong zone — no-op.
+	other := &Expedition{ZoneID: ZoneGoblinWarrens, TemporalStack: 5}
+	if reduceUnderforgeHeat(other) != 5 {
+		t.Error("reduceUnderforgeHeat on non-Underforge should be no-op")
+	}
+}
+
 func TestSunkenTemple_NoTidalOnNonZone(t *testing.T) {
 	setupZoneRunTestDB(t)
 	uid := id.UserID("@exp-tidal-other:example")