diff --git a/Nightborn.L0.sln b/Nightborn.L0.sln
index a1faa7c..11b2d22 100644
--- a/Nightborn.L0.sln
+++ b/Nightborn.L0.sln
@@ -7,6 +7,10 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{C3AD2FCF-F06
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Nightborn.Core", "src\Nightborn.Core\Nightborn.Core.csproj", "{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}"
 EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "tests", "tests", "{494AACD4-6553-4847-973E-DA41F0DC390B}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Nightborn.Core.Tests", "tests\Nightborn.Core.Tests\Nightborn.Core.Tests.csproj", "{B008EA0C-2A5E-456F-969C-1F9947084745}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -20,8 +24,13 @@ Global
 		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9}.Release|Any CPU.Build.0 = Release|Any CPU
+		{B008EA0C-2A5E-456F-969C-1F9947084745}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B008EA0C-2A5E-456F-969C-1F9947084745}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B008EA0C-2A5E-456F-969C-1F9947084745}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B008EA0C-2A5E-456F-969C-1F9947084745}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
 		{A1AFC50B-D6E5-4542-874A-2C911D6D3FE9} = {C3AD2FCF-F06D-48D2-8A84-2D8E73352FAD}
+		{B008EA0C-2A5E-456F-969C-1F9947084745} = {494AACD4-6553-4847-973E-DA41F0DC390B}
 	EndGlobalSection
 EndGlobal
diff --git a/src/Nightborn.Core/Combat/BloodEnergyConfig.cs b/src/Nightborn.Core/Combat/BloodEnergyConfig.cs
new file mode 100644
index 0000000..d59cc77
--- /dev/null
+++ b/src/Nightborn.Core/Combat/BloodEnergyConfig.cs
@@ -0,0 +1,16 @@
+namespace Nightborn.Core.Combat
+{
+    /// <summary>
+    /// Tunable parameters for blood energy formulas.
+    /// Values are based on GDD1 (Blood Energy Economy).
+    /// </summary>
+    public sealed class BloodEnergyConfig
+    {
+        public float BaseAttackGain { get; set; } = 5f;
+        public float BaseKillGain { get; set; } = 20f;
+        public float BaseHurtGain { get; set; } = 12f;
+        public float BaseSwitchCost { get; set; } = 40f;
+        public float BaseMaxEnergy { get; set; } = 100f;
+        public float FixedWaveBonus { get; set; } = 30f;
+    }
+}
diff --git a/src/Nightborn.Core/Combat/BloodEnergyEconomy.cs b/src/Nightborn.Core/Combat/BloodEnergyEconomy.cs
new file mode 100644
index 0000000..7d2805a
--- /dev/null
+++ b/src/Nightborn.Core/Combat/BloodEnergyEconomy.cs
@@ -0,0 +1,188 @@
+using System;
+
+namespace Nightborn.Core.Combat
+{
+    /// <summary>
+    /// L0 blood energy resource system.
+    /// - Pure C# (no Unity dependency)
+    /// - Handles gain/spend/reset/clamp rules
+    /// - Exposes formula helpers defined in GDD1
+    /// </summary>
+    public sealed class BloodEnergyEconomy
+    {
+        private const float MinimumSwitchCost = 1f;
+        private const float MaxSkillDiscount = 1f;
+
+        private readonly BloodEnergyConfig _config;
+        private float _current;
+        private float _max;
+        private bool _isDead;
+
+        public BloodEnergyEconomy(BloodEnergyConfig? config = null)
+        {
+            _config = config ?? new BloodEnergyConfig();
+            _max = _config.BaseMaxEnergy;
+            _current = 0f;
+            _isDead = false;
+        }
+
+        public float Current => _current;
+
+        public float Max => _max;
+
+        public bool IsDead => _isDead;
+
+        public event Action<float, float>? OnEnergyChanged;
+        public event Action? OnEnergyEmpty;
+        public event Action? OnEnergyFull;
+
+        public void SetDead(bool isDead)
+        {
+            _isDead = isDead;
+        }
+
+        public void Reset()
+        {
+            var previous = _current;
+            _current = 0f;
+            EmitChanged(previous, _current);
+        }
+
+        public void Add(float amount)
+        {
+            if (_isDead)
+            {
+                return;
+            }
+
+            if (amount <= 0f)
+            {
+                return;
+            }
+
+            var previous = _current;
+            _current = Clamp(previous + amount, 0f, _max);
+            EmitChanged(previous, _current);
+        }
+
+        public bool CanSpend(float amount)
+        {
+            return amount > 0f && _current >= amount;
+        }
+
+        public bool Spend(float amount)
+        {
+            if (!CanSpend(amount))
+            {
+                return false;
+            }
+
+            var previous = _current;
+            _current = Clamp(previous - amount, 0f, _max);
+            EmitChanged(previous, _current);
+            return true;
+        }
+
+        /// <summary>
+        /// Applies same-frame gain/spend with GDD order: gains first, then spend.
+        /// Returns true when spend succeeds.
+        /// </summary>
+        public bool ProcessFrame(float gainAmount, float spendAmount)
+        {
+            Add(gainAmount);
+            return Spend(spendAmount);
+        }
+
+        /// <summary>
+        /// Recomputes max energy by skill upgrade and clamps current if needed.
+        /// </summary>
+        public void SetUpgradeBonus(float upgradeBonus)
+        {
+            var previous = _current;
+            var safeUpgrade = Math.Max(0f, upgradeBonus);
+            _max = _config.BaseMaxEnergy * (1f + safeUpgrade);
+            _current = Clamp(_current, 0f, _max);
+            EmitChanged(previous, _current);
+        }
+
+        public float CalculateAttackGain(float enemyTypeMultiplier)
+        {
+            return Math.Max(0f, _config.BaseAttackGain * enemyTypeMultiplier);
+        }
+
+        public float CalculateKillGain(float enemyTypeMultiplier)
+        {
+            return Math.Max(0f, _config.BaseKillGain * enemyTypeMultiplier);
+        }
+
+        public float CalculateHurtGain(float damageTaken, float maxHealth)
+        {
+            if (maxHealth <= 0f || damageTaken <= 0f)
+            {
+                return 0f;
+            }
+
+            var ratio = Clamp(damageTaken / maxHealth, 0f, 1f);
+            return _config.BaseHurtGain * ratio;
+        }
+
+        public float CalculateSwitchCost(float skillDiscount)
+        {
+            var clampedDiscount = Clamp(skillDiscount, 0f, MaxSkillDiscount);
+            var cost = _config.BaseSwitchCost * (1f - clampedDiscount);
+            return Math.Max(cost, MinimumSwitchCost);
+        }
+
+        public float CalculateWaveClearBonus(int waveNumber)
+        {
+            if (waveNumber <= 0)
+            {
+                return 0f;
+            }
+
+            var raw = _config.FixedWaveBonus * waveNumber;
+            var cap = _config.FixedWaveBonus * 2f;
+            return Math.Min(raw, cap);
+        }
+
+        private void EmitChanged(float previous, float current)
+        {
+            if (Approximately(previous, current))
+            {
+                return;
+            }
+
+            OnEnergyChanged?.Invoke(_current, _max);
+
+            if (_current <= 0f)
+            {
+                OnEnergyEmpty?.Invoke();
+            }
+
+            if (Approximately(_current, _max))
+            {
+                OnEnergyFull?.Invoke();
+            }
+        }
+
+        private static float Clamp(float value, float min, float max)
+        {
+            if (value < min)
+            {
+                return min;
+            }
+
+            if (value > max)
+            {
+                return max;
+            }
+
+            return value;
+        }
+
+        private static bool Approximately(float a, float b)
+        {
+            return Math.Abs(a - b) < 0.0001f;
+        }
+    }
+}
diff --git a/tests/Nightborn.Core.Tests/BloodEnergyEconomyTests.cs b/tests/Nightborn.Core.Tests/BloodEnergyEconomyTests.cs
new file mode 100644
index 0000000..f6efb32
--- /dev/null
+++ b/tests/Nightborn.Core.Tests/BloodEnergyEconomyTests.cs
@@ -0,0 +1,188 @@
+using Nightborn.Core.Combat;
+using NUnit.Framework;
+
+namespace Nightborn.Core.Tests;
+
+public class BloodEnergyEconomyTests
+{
+    [Test]
+    public void AttackGain_ShouldIncreaseEnergy()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(sut.CalculateAttackGain(1f));
+
+        Assert.That(sut.Current, Is.EqualTo(5f).Within(0.0001f));
+    }
+
+    [Test]
+    public void Spend_WhenEnoughEnergy_ShouldSucceed()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(50f);
+
+        var ok = sut.Spend(40f);
+
+        Assert.That(ok, Is.True);
+        Assert.That(sut.Current, Is.EqualTo(10f).Within(0.0001f));
+    }
+
+    [Test]
+    public void Spend_WhenInsufficientEnergy_ShouldFailWithoutSideEffects()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(10f);
+
+        var ok = sut.Spend(40f);
+
+        Assert.That(ok, Is.False);
+        Assert.That(sut.Current, Is.EqualTo(10f).Within(0.0001f));
+    }
+
+    [Test]
+    public void Add_ShouldClampToMaxAndFireFullEvent()
+    {
+        var sut = new BloodEnergyEconomy();
+        var fullFired = false;
+        sut.OnEnergyFull += () => fullFired = true;
+
+        sut.Add(95f);
+        sut.Add(10f);
+
+        Assert.That(sut.Current, Is.EqualTo(100f).Within(0.0001f));
+        Assert.That(fullFired, Is.True);
+    }
+
+    [Test]
+    public void Reset_ShouldSetEnergyToZero()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(60f);
+
+        sut.Reset();
+
+        Assert.That(sut.Current, Is.EqualTo(0f).Within(0.0001f));
+    }
+
+    [Test]
+    public void EnergyEmpty_ShouldFireWhenReachesZero()
+    {
+        var sut = new BloodEnergyEconomy();
+        var emptyFired = false;
+        sut.OnEnergyEmpty += () => emptyFired = true;
+
+        sut.Add(24f);
+        sut.Spend(24f);
+
+        Assert.That(emptyFired, Is.True);
+    }
+
+    [Test]
+    public void SwitchCost_ShouldRespectDiscountAndMinimumCost()
+    {
+        var sut = new BloodEnergyEconomy();
+
+        var discounted = sut.CalculateSwitchCost(0.4f);
+        var minimum = sut.CalculateSwitchCost(1.0f);
+
+        Assert.That(discounted, Is.EqualTo(24f).Within(0.0001f));
+        Assert.That(minimum, Is.EqualTo(1f).Within(0.0001f));
+    }
+
+    [Test]
+    public void UpgradeBonusReduction_ShouldClampCurrentToNewMax()
+    {
+        var cfg = new BloodEnergyConfig { BaseMaxEnergy = 100f };
+        var sut = new BloodEnergyEconomy(cfg);
+        sut.Add(80f);
+
+        cfg.BaseMaxEnergy = 50f;
+        sut.SetUpgradeBonus(0f);
+
+        Assert.That(sut.Max, Is.EqualTo(50f).Within(0.0001f));
+        Assert.That(sut.Current, Is.EqualTo(50f).Within(0.0001f));
+    }
+
+    [Test]
+    public void Add_WhenDead_ShouldBeIgnored()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.SetDead(true);
+
+        sut.Add(10f);
+
+        Assert.That(sut.Current, Is.EqualTo(0f).Within(0.0001f));
+    }
+
+    [Test]
+    public void ProcessFrame_ShouldApplyGainBeforeSpend()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(35f);
+
+        var spendOk = sut.ProcessFrame(gainAmount: 10f, spendAmount: 40f);
+
+        Assert.That(spendOk, Is.True);
+        Assert.That(sut.Current, Is.EqualTo(5f).Within(0.0001f));
+    }
+
+    [Test]
+    public void ProcessFrame_WhenGainStillInsufficient_ShouldFailSpend()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(20f);
+
+        var spendOk = sut.ProcessFrame(gainAmount: 5f, spendAmount: 40f);
+
+        Assert.That(spendOk, Is.False);
+        Assert.That(sut.Current, Is.EqualTo(25f).Within(0.0001f));
+    }
+
+    [Test]
+    public void WaveClearBonus_ShouldCapAtTwoTimesFixedBonus()
+    {
+        var cfg = new BloodEnergyConfig { FixedWaveBonus = 30f };
+        var sut = new BloodEnergyEconomy(cfg);
+
+        Assert.That(sut.CalculateWaveClearBonus(1), Is.EqualTo(30f).Within(0.0001f));
+        Assert.That(sut.CalculateWaveClearBonus(2), Is.EqualTo(60f).Within(0.0001f));
+        Assert.That(sut.CalculateWaveClearBonus(10), Is.EqualTo(60f).Within(0.0001f));
+    }
+
+    [Test]
+    public void HurtGain_ShouldClampDamageRatioToOne()
+    {
+        var sut = new BloodEnergyEconomy(new BloodEnergyConfig { BaseHurtGain = 12f });
+
+        var gain = sut.CalculateHurtGain(damageTaken: 200f, maxHealth: 100f);
+
+        Assert.That(gain, Is.EqualTo(12f).Within(0.0001f));
+    }
+
+    [Test]
+    public void Add_NonPositiveAmount_ShouldNotChangeEnergyOrFireEvent()
+    {
+        var sut = new BloodEnergyEconomy();
+        var changedCount = 0;
+        sut.OnEnergyChanged += (_, _) => changedCount++;
+
+        sut.Add(0f);
+        sut.Add(-5f);
+
+        Assert.That(sut.Current, Is.EqualTo(0f).Within(0.0001f));
+        Assert.That(changedCount, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Spend_NonPositiveAmount_ShouldFailAndNotChangeEnergy()
+    {
+        var sut = new BloodEnergyEconomy();
+        sut.Add(10f);
+
+        var zero = sut.Spend(0f);
+        var negative = sut.Spend(-1f);
+
+        Assert.That(zero, Is.False);
+        Assert.That(negative, Is.False);
+        Assert.That(sut.Current, Is.EqualTo(10f).Within(0.0001f));
+    }
+}
diff --git a/tests/Nightborn.Core.Tests/GlobalUsings.cs b/tests/Nightborn.Core.Tests/GlobalUsings.cs
new file mode 100644
index 0000000..cefced4
--- /dev/null
+++ b/tests/Nightborn.Core.Tests/GlobalUsings.cs
@@ -0,0 +1 @@
+global using NUnit.Framework;
\ No newline at end of file
diff --git a/tests/Nightborn.Core.Tests/Nightborn.Core.Tests.csproj b/tests/Nightborn.Core.Tests/Nightborn.Core.Tests.csproj
new file mode 100644
index 0000000..c599966
--- /dev/null
+++ b/tests/Nightborn.Core.Tests/Nightborn.Core.Tests.csproj
@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.0" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.2.1" />
+    <PackageReference Include="NUnit.Analyzers" Version="3.6.1" />
+    <PackageReference Include="coverlet.collector" Version="6.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Nightborn.Core\Nightborn.Core.csproj" />
+  </ItemGroup>
+
+</Project>