diff --git a/src/data/ability.ts b/src/data/ability.ts
index 90335a85d..dba3aa5dd 100644
--- a/src/data/ability.ts
+++ b/src/data/ability.ts
@@ -2562,6 +2562,26 @@ export class IgnoreTypeImmunityAbAttr extends AbAttr {
   }
 }
 
+export class FullHpTypeMultiplierAbAttr extends AbAttr {
+  public multiplier: number;
+
+  constructor(multiplier: number) {
+    super(true);
+
+    this.multiplier = multiplier;
+  }
+
+  apply(pokemon: Pokemon, passive: boolean, cancelled: Utils.BooleanHolder, args: any[]): boolean {
+    const typeMultiplier = args[0] as Utils.NumberHolder;
+    if (typeMultiplier.value > 0){
+      typeMultiplier.value = this.multiplier;
+      return true;
+    }
+
+    return false;
+  }
+}
+
 function applyAbAttrsInternal<TAttr extends AbAttr>(attrType: { new(...args: any[]): TAttr },
   pokemon: Pokemon, applyFunc: AbAttrApplyFunc<TAttr>, args: any[], isAsync: boolean = false, showAbilityInstant: boolean = false, quiet: boolean = false, passive: boolean = false): Promise<void> {
   return new Promise(resolve => {
@@ -3707,10 +3727,10 @@ export function initAbilities() {
       .attr(NoTransformAbilityAbAttr)
       .attr(NoFusionAbilityAbAttr),
     new Ability(Abilities.TERA_SHELL, 9)
+      .attr(FullHpTypeMultiplierAbAttr, 0.5)
       .attr(UncopiableAbilityAbAttr)
       .attr(UnswappableAbilityAbAttr)
-      .ignorable()
-      .unimplemented(),
+      .ignorable(),
     new Ability(Abilities.TERAFORM_ZERO, 9)
       .attr(UncopiableAbilityAbAttr)
       .attr(UnswappableAbilityAbAttr)
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index a73f14586..63e9a10b7 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -27,7 +27,7 @@ import { TempBattleStat } from '../data/temp-battle-stat';
 import { ArenaTagSide, WeakenMoveScreenTag, WeakenMoveTypeTag } from '../data/arena-tag';
 import { ArenaTagType } from "../data/enums/arena-tag-type";
 import { Biome } from "../data/enums/biome";
-import { Ability, AbAttr, BattleStatMultiplierAbAttr, BlockCritAbAttr, BonusCritAbAttr, BypassBurnDamageReductionAbAttr, FieldPriorityMoveImmunityAbAttr, FieldVariableMovePowerAbAttr, IgnoreOpponentStatChangesAbAttr, MoveImmunityAbAttr, MoveTypeChangeAttr, NonSuperEffectiveImmunityAbAttr, PreApplyBattlerTagAbAttr, PreDefendFullHpEndureAbAttr, ReceivedMoveDamageMultiplierAbAttr, ReduceStatusEffectDurationAbAttr, StabBoostAbAttr, StatusEffectImmunityAbAttr, TypeImmunityAbAttr, VariableMovePowerAbAttr, VariableMoveTypeAbAttr, WeightMultiplierAbAttr, allAbilities, applyAbAttrs, applyBattleStatMultiplierAbAttrs, applyPostDefendAbAttrs, applyPreApplyBattlerTagAbAttrs, applyPreAttackAbAttrs, applyPreDefendAbAttrs, applyPreSetStatusAbAttrs, UnsuppressableAbilityAbAttr, SuppressFieldAbilitiesAbAttr, NoFusionAbilityAbAttr, MultCritAbAttr, IgnoreTypeImmunityAbAttr } from '../data/ability';
+import { Ability, AbAttr, BattleStatMultiplierAbAttr, BlockCritAbAttr, BonusCritAbAttr, BypassBurnDamageReductionAbAttr, FieldPriorityMoveImmunityAbAttr, FieldVariableMovePowerAbAttr, IgnoreOpponentStatChangesAbAttr, MoveImmunityAbAttr, MoveTypeChangeAttr, NonSuperEffectiveImmunityAbAttr, PreApplyBattlerTagAbAttr, PreDefendFullHpEndureAbAttr, ReceivedMoveDamageMultiplierAbAttr, ReduceStatusEffectDurationAbAttr, StabBoostAbAttr, StatusEffectImmunityAbAttr, TypeImmunityAbAttr, VariableMovePowerAbAttr, VariableMoveTypeAbAttr, WeightMultiplierAbAttr, allAbilities, applyAbAttrs, applyBattleStatMultiplierAbAttrs, applyPostDefendAbAttrs, applyPreApplyBattlerTagAbAttrs, applyPreAttackAbAttrs, applyPreDefendAbAttrs, applyPreSetStatusAbAttrs, UnsuppressableAbilityAbAttr, SuppressFieldAbilitiesAbAttr, NoFusionAbilityAbAttr, MultCritAbAttr, IgnoreTypeImmunityAbAttr, FullHpTypeMultiplierAbAttr } from '../data/ability';
 import { Abilities } from "#app/data/enums/abilities";
 import PokemonData from '../system/pokemon-data';
 import Battle, { BattlerIndex } from '../battle';
@@ -1442,6 +1442,8 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
     applyMoveAttrs(VariableMoveTypeMultiplierAttr, source, this, move, typeMultiplier);
     if (typeless)
       typeMultiplier.value = 1;
+    if (this.turnData.startedFullHp)
+      applyAbAttrs(FullHpTypeMultiplierAbAttr, this, null, typeMultiplier);
     if (types.find(t => move.isTypeImmune(t)))
       typeMultiplier.value = 0;
 
@@ -3310,6 +3312,7 @@ export class PokemonTurnData {
   public damageDealt: integer = 0;
   public damageTaken: integer = 0;
   public attacksReceived: AttackMoveResult[] = [];
+  public startedFullHp: boolean;
 }
 
 export enum AiType {
diff --git a/src/phases.ts b/src/phases.ts
index d8ce55e95..603528ad8 100644
--- a/src/phases.ts
+++ b/src/phases.ts
@@ -2433,6 +2433,9 @@ export class MoveEffectPhase extends PokemonPhase {
         if (this.move.getMove() instanceof AttackMove && !this.move.getMove().getAttrs(FixedDamageAttr).length)
           this.scene.applyModifiers(PokemonMultiHitModifier, user.isPlayer(), user, hitCount, new Utils.IntegerHolder(0));
         user.turnData.hitsLeft = user.turnData.hitCount = hitCount.value;
+
+        // Store if the target was full HP at the start of the hits
+        targets.map((t) => { t.turnData.startedFullHp = t.getHpRatio() === 1});
       }
 
       const moveHistoryEntry = { move: this.move.moveId, targets: this.targets, result: MoveResult.PENDING, virtual: this.move.virtual };
@@ -2537,6 +2540,9 @@ export class MoveEffectPhase extends PokemonPhase {
         if (hitsTotal > 1)
           this.scene.queueMessage(i18next.t('battle:attackHitsCount', { count: hitsTotal }));
         this.scene.applyModifiers(HitHealModifier, this.player, user);
+
+        // Update full HP check only after all hits
+        this.getTargets().map((t) => { t.turnData.startedFullHp = t.getHpRatio() === 1});
       }
     }