diff --git a/apps/api/src/combat/combat-engine.service.spec.ts b/apps/api/src/combat/combat-engine.service.spec.ts
index 6fe6c50..6ad59c7 100644
--- a/apps/api/src/combat/combat-engine.service.spec.ts
+++ b/apps/api/src/combat/combat-engine.service.spec.ts
@@ -867,4 +867,89 @@ describe('CombatEngineService', () => {
       );
     });
   });
+
+  const HOUND_ABILITIES: MonsterAbilities = {
+    bleed: { roundInterval: 2, damagePerRound: 6, durationRounds: 2 },
+    enrage: { hpThresholdPercent: 35, damageMultiplier: 1.4 },
+  };
+
+  describe('enrage', () => {
+    it('turns aggressive once its HP crosses the threshold', () => {
+      // 80 max HP, 40 left; a 14-damage blow lands it on 26, below 35%.
+      const state = baseState({
+        round: 1,
+        player: {
+          currentHp: 100,
+          maxHp: 100,
+          stats: { attack: 6, weaponDamage: 8, armor: 0 },
+        },
+        monster: withAbilities(HOUND_ABILITIES, {
+          currentHp: 40,
+          maxHp: 80,
+          stats: { attack: 12, armor: 0, abilities: HOUND_ABILITIES },
+        }),
+      });
+
+      const result = new CombatEngineService().resolveAction(state, {
+        action: CombatAction.ATTACK,
+      });
+
+      expect(result.state.monster.stats.enraged).toBe(true);
+      expect(result.events.map((event) => event.type)).toContain(
+        CombatEventType.ENRAGED,
+      );
+      // The same round's reply already hits harder: 12 * 1.4 = 16.8 -> 17.
+      const monsterHit = result.events.find(
+        (event) =>
+          event.type === CombatEventType.DAMAGE &&
+          event.source === Combatant.MONSTER,
+      );
+      expect(monsterHit?.amount).toBe(17);
+    });
+
+    it('stays quiet above the threshold', () => {
+      const state = baseState({
+        round: 1,
+        monster: withAbilities(HOUND_ABILITIES, {
+          currentHp: 80,
+          maxHp: 80,
+          stats: { attack: 12, armor: 0, abilities: HOUND_ABILITIES },
+        }),
+      });
+
+      const result = new CombatEngineService().resolveAction(state, {
+        action: CombatAction.ATTACK,
+      });
+
+      expect(result.state.monster.stats.enraged).toBeUndefined();
+      expect(result.events.map((event) => event.type)).not.toContain(
+        CombatEventType.ENRAGED,
+      );
+    });
+
+    it('announces the change only once', () => {
+      const state = baseState({
+        round: 2,
+        monster: withAbilities(HOUND_ABILITIES, {
+          currentHp: 20,
+          maxHp: 80,
+          stats: {
+            attack: 12,
+            armor: 0,
+            abilities: HOUND_ABILITIES,
+            enraged: true,
+          },
+        }),
+      });
+
+      const result = new CombatEngineService().resolveAction(state, {
+        action: CombatAction.DEFEND,
+      });
+
+      expect(result.events.map((event) => event.type)).not.toContain(
+        CombatEventType.ENRAGED,
+      );
+      expect(result.state.monster.stats.enraged).toBe(true);
+    });
+  });
 });
diff --git a/apps/api/src/combat/combat-engine.service.ts b/apps/api/src/combat/combat-engine.service.ts
index defe3a0..5050183 100644
--- a/apps/api/src/combat/combat-engine.service.ts
+++ b/apps/api/src/combat/combat-engine.service.ts
@@ -249,6 +249,8 @@ export class CombatEngineService {
     defended: boolean,
     events: CombatEngineEvent[],
   ): void {
+    this.checkEnrage(monster, events);
+
     const defendMultiplier = defended ? DEFEND_MITIGATION_MULTIPLIER : 1;
     const abilities = monster.stats.abilities ?? {};
 
@@ -305,10 +307,13 @@ export class CombatEngineService {
     multiplier: number,
     events: CombatEngineEvent[],
   ): void {
+    const enrageMultiplier = monster.stats.enraged
+      ? (monster.stats.abilities?.enrage?.damageMultiplier ?? 1)
+      : 1;
     const damage = calculateDamage(
       monster.stats,
       player.stats.armor,
-      multiplier,
+      multiplier * enrageMultiplier,
     );
     player.currentHp = Math.max(0, player.currentHp - damage);
     events.push({
@@ -319,6 +324,33 @@ export class CombatEngineService {
     });
   }
 
+  /**
+   * Latches the enraged state the first time the monster's HP crosses its
+   * threshold. Checked before it acts, so the blow that wounded it is already
+   * answered in kind.
+   */
+  private checkEnrage(
+    monster: CombatEngineCombatant,
+    events: CombatEngineEvent[],
+  ): void {
+    const enrage = monster.stats.abilities?.enrage;
+    if (!enrage || monster.stats.enraged) {
+      return;
+    }
+
+    const threshold = (monster.maxHp * enrage.hpThresholdPercent) / 100;
+    if (monster.currentHp > threshold) {
+      return;
+    }
+
+    monster.stats.enraged = true;
+    events.push({
+      source: Combatant.MONSTER,
+      target: Combatant.MONSTER,
+      type: CombatEventType.ENRAGED,
+    });
+  }
+
   /** Armor the monster actually presents this round, guard included. */
   private effectiveArmor(combatant: CombatEngineCombatant): number {
     return (
diff --git a/apps/api/src/combat/combat-engine.types.ts b/apps/api/src/combat/combat-engine.types.ts
index 421274a..cce3bed 100644
--- a/apps/api/src/combat/combat-engine.types.ts
+++ b/apps/api/src/combat/combat-engine.types.ts
@@ -44,6 +44,8 @@ export interface CombatEngineCombatantStats {
   // Effects currently ticking on this combatant. Only the player carries
   // any today -- nothing in this slice bleeds a monster.
   statusEffects?: ActiveStatusEffect[];
+  // Monster-only: latched the first time its HP crosses the enrage threshold.
+  enraged?: boolean;
 }
 
 export interface CombatEngineCombatant {
diff --git a/apps/api/src/combat/combat-event-type.enum.ts b/apps/api/src/combat/combat-event-type.enum.ts
index 1581e28..913195b 100644
--- a/apps/api/src/combat/combat-event-type.enum.ts
+++ b/apps/api/src/combat/combat-event-type.enum.ts
@@ -6,6 +6,7 @@ export enum CombatEventType {
   INTERRUPT = 'INTERRUPT',
   GUARD_RAISED = 'GUARD_RAISED',
   GUARD_ENDED = 'GUARD_ENDED',
+  ENRAGED = 'ENRAGED',
   STATUS_APPLIED = 'STATUS_APPLIED',
   STATUS_DAMAGE = 'STATUS_DAMAGE',
   STATUS_EXPIRED = 'STATUS_EXPIRED',
diff --git a/apps/api/src/monsters/monster-abilities.ts b/apps/api/src/monsters/monster-abilities.ts
index 195236e..ac24108 100644
--- a/apps/api/src/monsters/monster-abilities.ts
+++ b/apps/api/src/monsters/monster-abilities.ts
@@ -38,10 +38,21 @@ export interface MonsterGuardAbility {
   durationRounds: number;
 }
 
+export interface MonsterEnrageAbility {
+  /**
+   * Once its HP first falls to or below this share of maximum, the monster
+   * hits harder for the rest of the fight. One deterministic state change,
+   * never reversed and never rolled (Playable Slice 0.10 §5).
+   */
+  hpThresholdPercent: number;
+  damageMultiplier: number;
+}
+
 export interface MonsterAbilities {
   telegraph?: MonsterTelegraphAbility;
   bleed?: MonsterBleedAbility;
   guard?: MonsterGuardAbility;
+  enrage?: MonsterEnrageAbility;
 }
 
 export const NO_MONSTER_ABILITIES: MonsterAbilities = {};