diff --git a/.Lib9c.Tests/Model/RuneSkillOverflowTest.cs b/.Lib9c.Tests/Model/RuneSkillOverflowTest.cs
new file mode 100644
index 0000000000..c8b2da4cd6
--- /dev/null
+++ b/.Lib9c.Tests/Model/RuneSkillOverflowTest.cs
@@ -0,0 +1,148 @@
+namespace Lib9c.Tests.Model
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using Lib9c.Tests.Action;
+    using Nekoyume.Action;
+    using Nekoyume.Arena;
+    using Nekoyume.Helper;
+    using Nekoyume.Model;
+    using Nekoyume.Model.EnumType;
+    using Nekoyume.Model.Rune;
+    using Nekoyume.Model.Stat;
+    using Nekoyume.Model.State;
+    using Xunit;
+
+    /// <summary>
+    /// Regression test for PLD-1390. A caster-stat-referencing rune skill
+    /// (e.g. rune 10003, ATK% Caster) used to throw an <see cref="OverflowException"/>
+    /// inside <see cref="Player.SetRuneSkills"/> when <c>stat * SkillValue</c> exceeded
+    /// <see cref="int.MaxValue"/>, because the resulting power was cast to <c>int</c>.
+    /// The arena counterpart (<see cref="ArenaCharacter.SetRuneSkills"/>) clamped the same
+    /// value to <see cref="int.MaxValue"/> instead. Both now compute the power as
+    /// <c>long</c>, matching the rest of the skill/damage pipeline
+    /// (<see cref="Nekoyume.Model.Skill.SkillFactory"/>, <c>Skill.Power</c>,
+    /// <c>SkillCustomField.BuffValue</c> are all <c>long</c>).
+    /// Based on the real failing transaction
+    /// 1ab27944f05a195c1d5a1906513bd15e7c2e97253ba077e9bd0272965abf556a.
+    /// </summary>
+    public class RuneSkillOverflowTest
+    {
+        private const int RuneId = 10003;
+        private const int RuneLevel = 200;
+
+        // Values taken from the real failing tx: ATK 629,550,973, rune 10003 lv200
+        // (ATK% Caster, SkillValue 4.47). (int)Math.Round(629_550_973 * 4.47m) =
+        // 2,814,092,849 > int.MaxValue.
+        private const long Atk = 629_550_973L;
+        private const long ExpectedPower = 2_814_092_849L;
+
+        private readonly TableSheets _tableSheets;
+        private readonly AvatarState _avatarState;
+
+        public RuneSkillOverflowTest()
+        {
+            _tableSheets = new TableSheets(TableSheetsImporter.ImportSheets());
+            _avatarState = AvatarState.Create(
+                default,
+                default,
+                0,
+                _tableSheets.GetAvatarSheets(),
+                default);
+        }
+
+        [Fact]
+        public void SetRuneSkills_DoesNotOverflow_On_High_ATK()
+        {
+            var player = new Player(
+                _avatarState,
+                _tableSheets.CharacterSheet,
+                _tableSheets.CharacterLevelSheet,
+                _tableSheets.EquipmentItemSetEffectSheet);
+
+            // Inflate the player's ATK to the on-chain value via a collection Add modifier.
+            var baseAtk = player.Stats.GetStatAsLong(StatType.ATK);
+            player.Stats.SetCollections(new[]
+            {
+                new StatModifier(StatType.ATK, StatModifier.OperationType.Add, Atk - baseAtk),
+            });
+            Assert.Equal(Atk, player.Stats.GetStatAsLong(StatType.ATK));
+
+            // Sanity-check the sheet row matches the on-chain rune option.
+            Assert.True(
+                _tableSheets.RuneOptionSheet.TryGetOptionInfo(RuneId, RuneLevel, out var optionInfo));
+            Assert.Equal(4.47m, optionInfo.SkillValue);
+            Assert.Equal(StatModifier.OperationType.Percentage, optionInfo.SkillValueType);
+            Assert.Equal(StatType.ATK, optionInfo.SkillStatType);
+            Assert.Equal(StatReferenceType.Caster, optionInfo.StatReferenceType);
+
+            var runeState = new RuneState(RuneId, RuneLevel);
+
+            // Must not throw, and the power must be the full long value (no int truncation).
+            player.SetRuneSkills(
+                new List<RuneState> { runeState },
+                _tableSheets.RuneOptionSheet,
+                _tableSheets.SkillSheet);
+
+            var runeSkill = player.RuneSkills.Single();
+            Assert.Equal(ExpectedPower, runeSkill.Power);
+            Assert.True(runeSkill.CustomField.HasValue);
+            Assert.Equal(ExpectedPower, runeSkill.CustomField.Value.BuffValue);
+        }
+
+        [Fact]
+        public void ArenaCharacter_SetRuneSkills_KeepsFullLong_On_High_ATK()
+        {
+            // The arena path used to clamp the power to int.MaxValue via SafeDecimalToInt32;
+            // it now keeps the full long value so PvE and arena stay consistent.
+            var allRuneState = new AllRuneState();
+            allRuneState.AddRuneState(new RuneState(RuneId, RuneLevel));
+
+            // Rune 10003 is a Skill-type rune (rune_type 2) -> Skill slot (index 3).
+            var runeSlotState = new RuneSlotState(BattleType.Arena);
+            runeSlotState.UpdateSlot(
+                new List<RuneSlotInfo> { new RuneSlotInfo(3, RuneId) },
+                _tableSheets.RuneListSheet);
+
+            var digest = new ArenaPlayerDigest(
+                _avatarState,
+                new List<Nekoyume.Model.Item.Costume>(),
+                new List<Nekoyume.Model.Item.Equipment>(),
+                allRuneState,
+                runeSlotState);
+
+            var simulator = new ArenaSimulator(new TestRandom());
+            var character = new ArenaCharacter(
+                simulator,
+                digest,
+                _tableSheets.GetArenaSimulatorSheets(),
+                simulator.HpModifier,
+                new List<StatModifier>
+                {
+                    new StatModifier(StatType.ATK, StatModifier.OperationType.Add, Atk),
+                });
+
+            Assert.True(
+                _tableSheets.RuneOptionSheet.TryGetOptionInfo(RuneId, RuneLevel, out var optionInfo));
+            var expectedPower = (long)Math.Round(character.ATK * optionInfo.SkillValue);
+
+            var runeSkill = character._runeSkills.Single();
+            Assert.Equal(expectedPower, runeSkill.Power);
+            Assert.True(runeSkill.Power > int.MaxValue);
+        }
+
+        [Fact]
+        public void SafeDecimalToInt64_Preserves_Value_Where_Int_Cast_Overflows()
+        {
+            var rounded = Math.Round(629_550_973L * 4.47m);
+            Assert.Equal(2_814_092_849m, rounded);
+
+            // The old code path: an (int) cast of this decimal still overflow-checks and throws.
+            Assert.Throws<OverflowException>(() => (int)rounded);
+
+            // The new code path keeps the full value (well within long range).
+            Assert.Equal(2_814_092_849L, NumberConversionHelper.SafeDecimalToInt64(rounded));
+        }
+    }
+}
diff --git a/Lib9c/Model/Character/ArenaCharacter.cs b/Lib9c/Model/Character/ArenaCharacter.cs
index 774afca83f..d769f6b69b 100644
--- a/Lib9c/Model/Character/ArenaCharacter.cs
+++ b/Lib9c/Model/Character/ArenaCharacter.cs
@@ -470,16 +470,17 @@ public void SetRuneSkills(
                     continue;
                 }
 
-                var power = 0;
+                long power = 0;
 
                 if (optionInfo.SkillValueType == StatModifier.OperationType.Add)
                 {
-                    power = NumberConversionHelper.SafeDecimalToInt32(optionInfo.SkillValue);
+                    power = NumberConversionHelper.SafeDecimalToInt64(optionInfo.SkillValue);
                 }
                 else if (optionInfo.StatReferenceType == EnumType.StatReferenceType.Caster)
                 {
                     var value = Stats.GetStatAsLong(optionInfo.SkillStatType);
-                    power = NumberConversionHelper.SafeDecimalToInt32(Math.Round(value * optionInfo.SkillValue));
+                    power = NumberConversionHelper.SafeDecimalToInt64(
+                        Math.Round(value * optionInfo.SkillValue));
                 }
                 var skill = SkillFactory.GetForArena(skillRow, power, optionInfo.SkillChance, default, StatType.NONE);
                 var customField = new SkillCustomField
diff --git a/Lib9c/Model/Character/Player.cs b/Lib9c/Model/Character/Player.cs
index 80a888f7c0..8561a66161 100644
--- a/Lib9c/Model/Character/Player.cs
+++ b/Lib9c/Model/Character/Player.cs
@@ -3,6 +3,7 @@
 using System.Linq;
 using Nekoyume.Action;
 using Nekoyume.Battle;
+using Nekoyume.Helper;
 using Nekoyume.Model.BattleStatus;
 using Nekoyume.Model.Buff;
 using Nekoyume.Model.Item;
@@ -599,16 +600,17 @@ public void SetRuneSkills(
                     continue;
                 }
 
-                var power = 0;
+                long power = 0;
 
                 if (optionInfo.SkillValueType == StatModifier.OperationType.Add)
                 {
-                    power = (int)optionInfo.SkillValue;
+                    power = NumberConversionHelper.SafeDecimalToInt64(optionInfo.SkillValue);
                 }
                 else if (optionInfo.StatReferenceType == EnumType.StatReferenceType.Caster)
                 {
                     var value = Stats.GetStatAsLong(optionInfo.SkillStatType);
-                    power = (int)Math.Round(value * optionInfo.SkillValue);
+                    power = NumberConversionHelper.SafeDecimalToInt64(
+                        Math.Round(value * optionInfo.SkillValue));
                 }
                 var skill = SkillFactory.GetV1(skillRow, power, optionInfo.SkillChance);
                 var customField = new SkillCustomField