diff --git a/src/data/ability.ts b/src/data/ability.ts
index 3a14ad1456d..7d8220a629f 100644
--- a/src/data/ability.ts
+++ b/src/data/ability.ts
@@ -308,7 +308,7 @@ export class ClearWeatherAbAttr extends AbAttr {
 
   public override apply(pokemon: Pokemon, passive: boolean, simulated:boolean, cancelled: Utils.BooleanHolder, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetWeather(WeatherType.NONE, true);
+      globalScene.arena.trySetWeather(WeatherType.NONE, pokemon);
     }
   }
 }
@@ -334,7 +334,7 @@ export class ClearTerrainAbAttr extends AbAttr {
 
   public override apply(pokemon: Pokemon, passive: boolean, simulated:boolean, cancelled: Utils.BooleanHolder, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetTerrain(TerrainType.NONE, true, true);
+      globalScene.arena.trySetTerrain(TerrainType.NONE, true, pokemon);
     }
   }
 }
@@ -954,7 +954,7 @@ export class PostDefendTerrainChangeAbAttr extends PostDefendAbAttr {
 
   override applyPostDefend(pokemon: Pokemon, _passive: boolean, simulated: boolean, attacker: Pokemon, move: Move, hitResult: HitResult, _args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetTerrain(this.terrainType, true);
+      globalScene.arena.trySetTerrain(this.terrainType, false, pokemon);
     }
   }
 }
@@ -1126,7 +1126,7 @@ export class PostDefendWeatherChangeAbAttr extends PostDefendAbAttr {
 
   override applyPostDefend(pokemon: Pokemon, _passive: boolean, simulated: boolean, attacker: Pokemon, move: Move, _hitResult: HitResult, _args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetWeather(this.weatherType, true);
+      globalScene.arena.trySetWeather(this.weatherType, pokemon);
     }
   }
 }
@@ -2483,7 +2483,7 @@ export class PostSummonWeatherChangeAbAttr extends PostSummonAbAttr {
 
   override applyPostSummon(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetWeather(this.weatherType, true);
+      globalScene.arena.trySetWeather(this.weatherType, pokemon);
     }
   }
 }
@@ -2503,7 +2503,7 @@ export class PostSummonTerrainChangeAbAttr extends PostSummonAbAttr {
 
   override applyPostSummon(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetTerrain(this.terrainType, true);
+      globalScene.arena.trySetTerrain(this.terrainType, false, pokemon);
     }
   }
 }
@@ -2886,7 +2886,7 @@ export class PreSwitchOutClearWeatherAbAttr extends PreSwitchOutAbAttr {
     }
 
     if (turnOffWeather) {
-      globalScene.arena.trySetWeather(WeatherType.NONE, false);
+      globalScene.arena.trySetWeather(WeatherType.NONE);
       return true;
     }
 
@@ -2986,7 +2986,7 @@ export class PreLeaveFieldClearWeatherAbAttr extends PreLeaveFieldAbAttr {
    */
   override applyPreLeaveField(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetWeather(WeatherType.NONE, false);
+      globalScene.arena.trySetWeather(WeatherType.NONE);
     }
   }
 }
@@ -4137,7 +4137,7 @@ export class PostBiomeChangeWeatherChangeAbAttr extends PostBiomeChangeAbAttr {
 
   override apply(pokemon: Pokemon, passive: boolean, simulated: boolean, cancelled: Utils.BooleanHolder, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetWeather(this.weatherType, true);
+      globalScene.arena.trySetWeather(this.weatherType, pokemon);
     }
   }
 }
@@ -4157,7 +4157,7 @@ export class PostBiomeChangeTerrainChangeAbAttr extends PostBiomeChangeAbAttr {
 
   override apply(pokemon: Pokemon, passive: boolean, simulated: boolean, cancelled: Utils.BooleanHolder, args: any[]): void {
     if (!simulated) {
-      globalScene.arena.trySetTerrain(this.terrainType, true);
+      globalScene.arena.trySetTerrain(this.terrainType, false, pokemon);
     }
   }
 }
diff --git a/src/data/moves/move.ts b/src/data/moves/move.ts
index 07d621f6f2c..1af872e0915 100644
--- a/src/data/moves/move.ts
+++ b/src/data/moves/move.ts
@@ -2851,7 +2851,7 @@ export class WeatherChangeAttr extends MoveEffectAttr {
   }
 
   apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
-    return globalScene.arena.trySetWeather(this.weatherType, true);
+    return globalScene.arena.trySetWeather(this.weatherType, user);
   }
 
   getCondition(): MoveConditionFunc {
@@ -2870,7 +2870,7 @@ export class ClearWeatherAttr extends MoveEffectAttr {
 
   apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
     if (globalScene.arena.weather?.weatherType === this.weatherType) {
-      return globalScene.arena.trySetWeather(WeatherType.NONE, true);
+      return globalScene.arena.trySetWeather(WeatherType.NONE, user);
     }
 
     return false;
@@ -2887,7 +2887,7 @@ export class TerrainChangeAttr extends MoveEffectAttr {
   }
 
   apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
-    return globalScene.arena.trySetTerrain(this.terrainType, true, true);
+    return globalScene.arena.trySetTerrain(this.terrainType, true, user);
   }
 
   getCondition(): MoveConditionFunc {
@@ -2906,7 +2906,7 @@ export class ClearTerrainAttr extends MoveEffectAttr {
   }
 
   apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
-    return globalScene.arena.trySetTerrain(TerrainType.NONE, true, true);
+    return globalScene.arena.trySetTerrain(TerrainType.NONE, true, user);
   }
 }
 
@@ -6454,7 +6454,7 @@ export class ForceSwitchOutAttr extends MoveEffectAttr {
 
 export class ChillyReceptionAttr extends ForceSwitchOutAttr {
   apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
-    globalScene.arena.trySetWeather(WeatherType.SNOW, true);
+    globalScene.arena.trySetWeather(WeatherType.SNOW, user);
     return super.apply(user, target, move, args);
   }
 
diff --git a/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts b/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
index d9bd6983d97..6118fe3d0de 100644
--- a/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
+++ b/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
@@ -140,7 +140,8 @@ export const FieryFalloutEncounter: MysteryEncounter = MysteryEncounterBuilder.w
     // Load animations/sfx for Volcarona moves
     loadCustomMovesForEncounter([Moves.FIRE_SPIN, Moves.QUIVER_DANCE]);
 
-    globalScene.arena.trySetWeather(WeatherType.SUNNY, true);
+    const pokemon = globalScene.getEnemyPokemon();
+    globalScene.arena.trySetWeather(WeatherType.SUNNY, pokemon);
 
     encounter.setDialogueToken("volcaronaName", getPokemonSpecies(Species.VOLCARONA).getName());
 
diff --git a/src/field/arena.ts b/src/field/arena.ts
index 2c538de890f..4f243789567 100644
--- a/src/field/arena.ts
+++ b/src/field/arena.ts
@@ -42,6 +42,7 @@ import { SpeciesFormChangeRevertWeatherFormTrigger, SpeciesFormChangeWeatherTrig
 import { CommonAnimPhase } from "#app/phases/common-anim-phase";
 import { ShowAbilityPhase } from "#app/phases/show-ability-phase";
 import { WeatherType } from "#enums/weather-type";
+import { FieldEffectModifier } from "#app/modifier/modifier";
 
 export class Arena {
   public biomeType: Biome;
@@ -311,10 +312,10 @@ export class Arena {
   /**
    * Attempts to set a new weather to the battle
    * @param weather {@linkcode WeatherType} new {@linkcode WeatherType} to set
-   * @param hasPokemonSource boolean if the new weather is from a pokemon
+   * @param user {@linkcode Pokemon} that caused the weather effect
    * @returns true if new weather set, false if no weather provided or attempting to set the same weather as currently in use
    */
-  trySetWeather(weather: WeatherType, hasPokemonSource: boolean): boolean {
+  trySetWeather(weather: WeatherType, user?: Pokemon): boolean {
     if (Overrides.WEATHER_OVERRIDE) {
       return this.trySetWeatherOverride(Overrides.WEATHER_OVERRIDE);
     }
@@ -336,7 +337,14 @@ export class Arena {
       return false;
     }
 
-    this.weather = weather ? new Weather(weather, hasPokemonSource ? 5 : 0) : null;
+    const weatherDuration = new Utils.NumberHolder(0);
+
+    if (!Utils.isNullOrUndefined(user)) {
+      weatherDuration.value = 5;
+      globalScene.applyModifier(FieldEffectModifier, user.isPlayer(), user, weatherDuration);
+    }
+
+    this.weather = weather ? new Weather(weather, weatherDuration.value) : null;
     this.eventTarget.dispatchEvent(
       new WeatherChangedEvent(oldWeatherType, this.weather?.weatherType!, this.weather?.turnsLeft!),
     ); // TODO: is this bang correct?
@@ -398,14 +406,29 @@ export class Arena {
     return !(this.terrain?.terrainType === (terrain || undefined));
   }
 
-  trySetTerrain(terrain: TerrainType, hasPokemonSource: boolean, ignoreAnim = false): boolean {
+  /**
+   * Attempts to set a new terrain effect to the battle
+   * @param terrain {@linkcode TerrainType} new {@linkcode TerrainType} to set
+   * @param ignoreAnim boolean if the terrain animation should be ignored
+   * @param user {@linkcode Pokemon} that caused the terrain effect
+   * @returns true if new terrain set, false if no terrain provided or attempting to set the same terrain as currently in use
+   */
+  trySetTerrain(terrain: TerrainType, ignoreAnim = false, user?: Pokemon): boolean {
     if (!this.canSetTerrain(terrain)) {
       return false;
     }
 
     const oldTerrainType = this.terrain?.terrainType || TerrainType.NONE;
 
-    this.terrain = terrain ? new Terrain(terrain, hasPokemonSource ? 5 : 0) : null;
+    const terrainDuration = new Utils.NumberHolder(0);
+
+    if (!Utils.isNullOrUndefined(user)) {
+      terrainDuration.value = 5;
+      globalScene.applyModifier(FieldEffectModifier, user.isPlayer(), user, terrainDuration);
+    }
+
+    this.terrain = terrain ? new Terrain(terrain, terrainDuration.value) : null;
+
     this.eventTarget.dispatchEvent(
       new TerrainChangedEvent(oldTerrainType, this.terrain?.terrainType!, this.terrain?.turnsLeft!),
     ); // TODO: are those bangs correct?
@@ -802,9 +825,9 @@ export class Arena {
   resetArenaEffects(): void {
     // Don't reset weather if a Biome's permanent weather is active
     if (this.weather?.turnsLeft !== 0) {
-      this.trySetWeather(WeatherType.NONE, false);
+      this.trySetWeather(WeatherType.NONE);
     }
-    this.trySetTerrain(TerrainType.NONE, false, true);
+    this.trySetTerrain(TerrainType.NONE, true);
     this.resetPlayerFaintCount();
     this.removeAllTags();
   }
diff --git a/src/modifier/modifier-type.ts b/src/modifier/modifier-type.ts
index e8470853388..b5574c02c8b 100644
--- a/src/modifier/modifier-type.ts
+++ b/src/modifier/modifier-type.ts
@@ -97,6 +97,7 @@ import {
   type PersistentModifier,
   TempExtraModifierModifier,
   CriticalCatchChanceBoosterModifier,
+  FieldEffectModifier,
 } from "#app/modifier/modifier";
 import { ModifierTier } from "#app/modifier/modifier-tier";
 import Overrides from "#app/overrides";
@@ -1998,6 +1999,13 @@ export const modifierTypes = {
       return new PokemonNatureChangeModifierType(randSeedInt(getEnumValues(Nature).length) as Nature);
     }),
 
+  MYSTICAL_ROCK: () =>
+    new ModifierType(
+      "modifierType:ModifierType.MYSTICAL_ROCK",
+      "mystical_rock",
+      (type, args) => new FieldEffectModifier(type, (args[0] as Pokemon).id),
+    ),
+
   TERA_SHARD: () =>
     new ModifierTypeGenerator((party: Pokemon[], pregenArgs?: any[]) => {
       if (pregenArgs && pregenArgs.length === 1 && pregenArgs[0] in PokemonType) {
@@ -2810,6 +2818,47 @@ const modifierPool: ModifierPool = {
       },
       10,
     ),
+    new WeightedModifierType(
+      modifierTypes.MYSTICAL_ROCK,
+      (party: Pokemon[]) => {
+        return party.some(p => {
+          const moveset = p.getMoveset(true).map(m => m.moveId);
+
+          const hasAbility = [
+            Abilities.DRIZZLE,
+            Abilities.ORICHALCUM_PULSE,
+            Abilities.DRIZZLE,
+            Abilities.SAND_STREAM,
+            Abilities.SAND_SPIT,
+            Abilities.SNOW_WARNING,
+            Abilities.ELECTRIC_SURGE,
+            Abilities.HADRON_ENGINE,
+            Abilities.PSYCHIC_SURGE,
+            Abilities.GRASSY_SURGE,
+            Abilities.SEED_SOWER,
+            Abilities.MISTY_SURGE,
+          ].some(a => p.hasAbility(a, false, true));
+
+          const hasMoves = [
+            Moves.SUNNY_DAY,
+            Moves.RAIN_DANCE,
+            Moves.SANDSTORM,
+            Moves.SNOWSCAPE,
+            Moves.HAIL,
+            Moves.CHILLY_RECEPTION,
+            Moves.ELECTRIC_TERRAIN,
+            Moves.PSYCHIC_TERRAIN,
+            Moves.GRASSY_TERRAIN,
+            Moves.MISTY_TERRAIN,
+          ].some(m => moveset.includes(m));
+
+          return hasAbility || hasMoves;
+        })
+          ? 10
+          : 0;
+      },
+      10,
+    ),
     new WeightedModifierType(modifierTypes.REVIVER_SEED, 4),
     new WeightedModifierType(modifierTypes.CANDY_JAR, skipInLastClassicWaveOrDefault(5)),
     new WeightedModifierType(modifierTypes.ATTACK_TYPE_BOOSTER, 9),
diff --git a/src/modifier/modifier.ts b/src/modifier/modifier.ts
index 4c59bfe1ef1..7c9207bbea5 100644
--- a/src/modifier/modifier.ts
+++ b/src/modifier/modifier.ts
@@ -2014,6 +2014,38 @@ export class ResetNegativeStatStageModifier extends PokemonHeldItemModifier {
   }
 }
 
+/**
+ * Modifier used for held items, namely Mystical Rock, that extend the
+ * duration of weather and terrain effects.
+ * @extends PokemonHeldItemModifier
+ * @see {@linkcode apply}
+ */
+export class FieldEffectModifier extends PokemonHeldItemModifier {
+  /**
+   * Provides two more turns per stack to any weather or terrain effect caused
+   * by the holder.
+   * @param pokemon {@linkcode Pokemon} that holds the held item
+   * @param fieldDuration {@linkcode NumberHolder} that stores the current field effect duration
+   * @returns `true` if the field effect extension was applied successfully
+   */
+  override apply(_pokemon: Pokemon, fieldDuration: NumberHolder): boolean {
+    fieldDuration.value += 2 * this.stackCount;
+    return true;
+  }
+
+  override matchType(modifier: Modifier): boolean {
+    return modifier instanceof FieldEffectModifier;
+  }
+
+  override clone(): FieldEffectModifier {
+    return new FieldEffectModifier(this.type, this.pokemonId, this.stackCount);
+  }
+
+  override getMaxHeldItemCount(_pokemon?: Pokemon): number {
+    return 2;
+  }
+}
+
 export abstract class ConsumablePokemonModifier extends ConsumableModifier {
   public pokemonId: number;
 
diff --git a/src/phases/encounter-phase.ts b/src/phases/encounter-phase.ts
index 5decab522b5..7afd4176788 100644
--- a/src/phases/encounter-phase.ts
+++ b/src/phases/encounter-phase.ts
@@ -684,7 +684,7 @@ export class EncounterPhase extends BattlePhase {
    */
   trySetWeatherIfNewBiome(): void {
     if (!this.loaded) {
-      globalScene.arena.trySetWeather(getRandomWeatherType(globalScene.arena), false);
+      globalScene.arena.trySetWeather(getRandomWeatherType(globalScene.arena));
     }
   }
 }
diff --git a/src/phases/new-biome-encounter-phase.ts b/src/phases/new-biome-encounter-phase.ts
index ed768742aac..bb1fe54fe9f 100644
--- a/src/phases/new-biome-encounter-phase.ts
+++ b/src/phases/new-biome-encounter-phase.ts
@@ -45,6 +45,6 @@ export class NewBiomeEncounterPhase extends NextEncounterPhase {
    * Set biome weather.
    */
   trySetWeatherIfNewBiome(): void {
-    globalScene.arena.trySetWeather(getRandomWeatherType(globalScene.arena), false);
+    globalScene.arena.trySetWeather(getRandomWeatherType(globalScene.arena));
   }
 }
diff --git a/src/phases/turn-end-phase.ts b/src/phases/turn-end-phase.ts
index c55f6d69a58..836647fbfb4 100644
--- a/src/phases/turn-end-phase.ts
+++ b/src/phases/turn-end-phase.ts
@@ -68,12 +68,12 @@ export class TurnEndPhase extends FieldPhase {
     globalScene.arena.lapseTags();
 
     if (globalScene.arena.weather && !globalScene.arena.weather.lapse()) {
-      globalScene.arena.trySetWeather(WeatherType.NONE, false);
+      globalScene.arena.trySetWeather(WeatherType.NONE);
       globalScene.arena.triggerWeatherBasedFormChangesToNormal();
     }
 
     if (globalScene.arena.terrain && !globalScene.arena.terrain.lapse()) {
-      globalScene.arena.trySetTerrain(TerrainType.NONE, false);
+      globalScene.arena.trySetTerrain(TerrainType.NONE);
     }
 
     this.end();
diff --git a/test/abilities/forecast.test.ts b/test/abilities/forecast.test.ts
index bdc32604bba..642b490da72 100644
--- a/test/abilities/forecast.test.ts
+++ b/test/abilities/forecast.test.ts
@@ -181,7 +181,7 @@ describe("Abilities - Forecast", () => {
 
       expect(castform.formIndex).toBe(SNOWY_FORM);
 
-      game.scene.arena.trySetWeather(WeatherType.FOG, false);
+      game.scene.arena.trySetWeather(WeatherType.FOG);
       game.move.select(Moves.SPLASH);
       game.move.select(Moves.SPLASH, 1);
       await game.phaseInterceptor.to("TurnStartPhase");
diff --git a/test/items/mystical_rock.test.ts b/test/items/mystical_rock.test.ts
new file mode 100644
index 00000000000..0558bc21fe1
--- /dev/null
+++ b/test/items/mystical_rock.test.ts
@@ -0,0 +1,60 @@
+import { globalScene } from "#app/global-scene";
+import { Moves } from "#enums/moves";
+import { Abilities } from "#enums/abilities";
+import { Species } from "#enums/species";
+import GameManager from "#test/testUtils/gameManager";
+import Phase from "phaser";
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
+
+describe("Items - Mystical Rock", () => {
+  let phaserGame: Phaser.Game;
+  let game: GameManager;
+
+  beforeAll(() => {
+    phaserGame = new Phase.Game({
+      type: Phaser.HEADLESS,
+    });
+  });
+
+  afterEach(() => {
+    game.phaseInterceptor.restoreOg();
+  });
+
+  beforeEach(() => {
+    game = new GameManager(phaserGame);
+
+    game.override
+      .enemySpecies(Species.SHUCKLE)
+      .enemyMoveset(Moves.SPLASH)
+      .enemyAbility(Abilities.BALL_FETCH)
+      .moveset([Moves.SUNNY_DAY, Moves.GRASSY_TERRAIN])
+      .startingHeldItems([{ name: "MYSTICAL_ROCK", count: 2 }])
+      .battleType("single");
+  });
+
+  it("should increase weather duration by +2 turns per stack", async () => {
+    await game.classicMode.startBattle([Species.GASTLY]);
+
+    game.move.select(Moves.SUNNY_DAY);
+
+    await game.phaseInterceptor.to("MoveEndPhase");
+
+    const weather = globalScene.arena.weather;
+
+    expect(weather).toBeDefined();
+    expect(weather!.turnsLeft).to.equal(9);
+  });
+
+  it("should increase terrain duration by +2 turns per stack", async () => {
+    await game.classicMode.startBattle([Species.GASTLY]);
+
+    game.move.select(Moves.GRASSY_TERRAIN);
+
+    await game.phaseInterceptor.to("MoveEndPhase");
+
+    const terrain = globalScene.arena.terrain;
+
+    expect(terrain).toBeDefined();
+    expect(terrain!.turnsLeft).to.equal(9);
+  });
+});
diff --git a/test/moves/dive.test.ts b/test/moves/dive.test.ts
index 8d7b0f9dd00..d7b53701a25 100644
--- a/test/moves/dive.test.ts
+++ b/test/moves/dive.test.ts
@@ -120,7 +120,7 @@ describe("Moves - Dive", () => {
 
     await game.phaseInterceptor.to("TurnEndPhase");
     await game.phaseInterceptor.to("TurnStartPhase", false);
-    game.scene.arena.trySetWeather(WeatherType.HARSH_SUN, false);
+    game.scene.arena.trySetWeather(WeatherType.HARSH_SUN);
 
     await game.phaseInterceptor.to("MoveEndPhase");
     expect(playerPokemon.getLastXMoves(1)[0].result).toBe(MoveResult.FAIL);