diff --git a/packages/engine/src/game.ts b/packages/engine/src/game.ts
index ea211e6..12bde4a 100644
--- a/packages/engine/src/game.ts
+++ b/packages/engine/src/game.ts
@@ -199,6 +199,8 @@ export interface CastParams {
   cardId?: string;
   points?: number;
   clockwise?: boolean;
+  /** MEGA-MONSTER: which stat doubles. */
+  boost?: "life" | "movement";
 }
 
 export interface GameConfig {
@@ -1465,7 +1467,7 @@ const CARD_EFFECTS: Record<string, AttackEffect | NeutralEffect | CounterEffect>
       if (!creature) return "no such monster";
       if (creature.kind === "shadow" || creature.kind === "alter-ego") return "that is no monster";
       if (!gameLos(state, caster.position, creature.position)) return "no line of sight";
-      const boost = cmd.params?.cardId === "movement" ? "movement" : "life";
+      const boost = cmd.params?.boost === "movement" ? "movement" : "life";
       if (boost === "movement") creature.movesPerTurn *= 2;
       else creature.maxDamage *= 2;
       events.push({ type: "monsterBoosted", creatureId: creature.id, boost });
diff --git a/packages/engine/test/creatures.test.ts b/packages/engine/test/creatures.test.ts
index e00d36a..01d6f79 100644
--- a/packages/engine/test/creatures.test.ts
+++ b/packages/engine/test/creatures.test.ts
@@ -205,6 +205,16 @@ describe("monsters", () => {
     });
     expect(state.creatures[0]!.maxDamage).toBe(8);
 
+    // "Doubles the existing life-points OR movement rate" — the other choice.
+    const movesBefore = state.creatures[0]!.movesPerTurn;
+    const mm2 = giveCard(state, me, "mega-monster", "MM2", 3);
+    state = must(state, me, {
+      type: "cast", instanceId: mm2.instanceId, target: { kind: "creature", creatureId: id },
+      params: { boost: "movement" },
+    });
+    expect(state.creatures[0]!.movesPerTurn).toBe(movesBefore * 2);
+    expect(state.creatures[0]!.maxDamage).toBe(8);
+
     const dc = giveCard(state, me, "dispel-creation", "DC", 2);
     state = must(state, me, {
       type: "cast", instanceId: dc.instanceId, target: { kind: "cell", cell: state.creatures[0]!.position },
diff --git a/packages/web/src/App.svelte b/packages/web/src/App.svelte
index 3bdf21b..e54e829 100644
--- a/packages/web/src/App.svelte
+++ b/packages/web/src/App.svelte
@@ -88,6 +88,8 @@
   let tradeFrom = $state<{ x: number; y: number } | null>(null);
   /** rotate-sector direction. */
   let rotateCW = $state(true);
+  /** mega-monster: which stat the chosen monster doubles. */
+  let megaBoost = $state<"life" | "movement">("life");
   /** Cards marked for discard. */
   let discardSelection = $state<Set<string>>(new Set());
   /** Voluntary discard mode: hand clicks mark cards instead of playing them. */
@@ -226,6 +228,7 @@
     tradeFrom = null;
     selectedCreature = null;
     selectedCard = null;
+    megaBoost = "life";
     attachedNumber = null;
     attachedMods = [];
     wbDamage = 0;
@@ -614,6 +617,7 @@
       dispatch({
         type: "cast", instanceId: selectedCard.instanceId,
         target: { kind: "creature", creatureId },
+        ...(selectedCard.cardId === "mega-monster" ? { params: { boost: megaBoost } } : {}),
         ...(attachedNumber ? { numberInstanceIds: [attachedNumber.instanceId] } : {}),
       });
       clearSelection();
@@ -1378,6 +1382,14 @@
             {:else if selectedCard && TWO_CELL_CARDS.has(selectedCard.cardId)}
               <span>{tradeFrom ? "— now the second square" : "— click the first square"}</span>
             {/if}
+            {#if selectedCard?.cardId === "mega-monster"}
+              <span>— double its</span>
+              <button class="stamp tiny" class:lit={megaBoost === "life"}
+                onclick={() => (megaBoost = "life")}>hit points</button>
+              <button class="stamp tiny" class:lit={megaBoost === "movement"}
+                onclick={() => (megaBoost = "movement")}>movement</button>
+              <span>— then tap the monster</span>
+            {/if}
             {#if selectedCard?.cardId === "rotate-sector"}
               <label class="inline"><input type="checkbox" bind:checked={rotateCW} /> clockwise</label>
               <span>— click the sector</span>
@@ -1857,6 +1869,10 @@
     cursor: pointer;
     box-shadow: 2px 2px 0 rgba(0, 0, 0, 0.35);
   }
+  .stamp.lit {
+    background: #43331f;
+    color: #e9e1cb;
+  }
   .stamp:hover:not(:disabled) { background: #f4eede; }
   .stamp:active:not(:disabled) { transform: translate(1px, 1px); box-shadow: 1px 1px 0 rgba(0,0,0,0.35); }
   .stamp:disabled { opacity: 0.45; cursor: default; }