diff --git a/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs b/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs
new file mode 100644
index 000000000..09941107c
--- /dev/null
+++ b/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs
@@ -0,0 +1,74 @@
+using ProjectM.Simulation;
+using Unity.Burst;
+using Unity.Collections;
+using Unity.Entities;
+using Unity.Mathematics;
+using Unity.NetCode;
+using Unity.Transforms;
+
+namespace ProjectM.Server
+{
+    /// <summary>
+    /// LANTERN "light is territory" relevancy prototype (Build Spec Phase 1 step 8): per connection, marks every
+    /// ENEMY ghost OUTSIDE that player's lamp radius IRRELEVANT — so monsters in the dark aren't replicated (the
+    /// gamma test: "nothing to see"). Players + untagged/global ghosts stay relevant for free (SetIsIrrelevant).
+    /// <para>
+    /// SHARED-SET DISCIPLINE: <see cref="RegionRelevancySystem"/> is the SOLE clearer of the <c>GhostRelevancy</c>
+    /// set — its OnUpdate runs every tick (requires only <c>GhostRelevancy</c>), setting the mode + clearing +
+    /// adding its region hides. This system runs <b>[UpdateAfter(RegionRelevancySystem)]</b> and only ADDS light
+    /// hides (never clears) via <c>TryAdd</c> (defensive against a ghost hidden by BOTH region and light for one
+    /// connection — a duplicate <c>Add</c> would throw). Gated on <see cref="GymTag"/> so it runs ONLY in the
+    /// LANTERN gym; the legacy base/expedition game keeps region relevancy alone. Runs in
+    /// <see cref="GhostSimulationSystemGroup"/> before GhostSendSystem reads the set. Per-player dim/bright (a
+    /// variable radius) is the tuning follow-up; the prototype uses <see cref="LightRelevancyMath.LampRadiusDefault"/>.
+    /// </para>
+    /// </summary>
+    [BurstCompile]
+    [WorldSystemFilter(WorldSystemFilterFlags.ServerSimulation)]
+    [UpdateInGroup(typeof(GhostSimulationSystemGroup))]
+    [UpdateAfter(typeof(RegionRelevancySystem))]
+    public partial struct LightRelevancySystem : ISystem
+    {
+        [BurstCompile]
+        public void OnCreate(ref SystemState state)
+        {
+            state.RequireForUpdate<GhostRelevancy>();
+            state.RequireForUpdate<GymTag>(); // prototype: light-as-territory runs only in the LANTERN gym
+        }
+
+        [BurstCompile]
+        public void OnUpdate(ref SystemState state)
+        {
+            // Each in-game connection's player position (per-player lamp radius = the dim/bright follow-up; fixed here).
+            var connPos = new NativeHashMap<int, float3>(8, Allocator.Temp);
+            foreach (var (owner, lt) in
+                     SystemAPI.Query<RefRO<GhostOwner>, RefRO<LocalTransform>>().WithAll<PlayerTag>())
+                connPos[owner.ValueRO.NetworkId] = lt.ValueRO.Position;
+
+            if (connPos.IsEmpty) { connPos.Dispose(); return; }
+
+            // The set is already mode=SetIsIrrelevant + cleared + region-populated THIS tick by RegionRelevancySystem.
+            ref var relevancy = ref SystemAPI.GetSingletonRW<GhostRelevancy>().ValueRW;
+            var set = relevancy.GhostRelevancySet;
+            var conns = connPos.GetKeyValueArrays(Allocator.Temp);
+
+            float rSq = LightRelevancyMath.LampRadiusDefault * LightRelevancyMath.LampRadiusDefault;
+            foreach (var (ghost, lt) in
+                     SystemAPI.Query<RefRO<GhostInstance>, RefRO<LocalTransform>>().WithAll<EnemyTag>())
+            {
+                int ghostId = ghost.ValueRO.ghostId;
+                if (ghostId == 0) continue; // ghost id not assigned yet this tick
+
+                float3 gp = lt.ValueRO.Position;
+                for (int i = 0; i < conns.Keys.Length; i++)
+                {
+                    if (LightRelevancyMath.IsHidden(conns.Values[i], gp, rSq))
+                        set.TryAdd(new RelevantGhostForConnection { Connection = conns.Keys[i], Ghost = ghostId }, 1);
+                }
+            }
+
+            conns.Dispose();
+            connPos.Dispose();
+        }
+    }
+}
diff --git a/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs.meta b/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs.meta
new file mode 100644
index 000000000..fb5ed0b7b
--- /dev/null
+++ b/Assets/_Project/Scripts/Server/World/LightRelevancySystem.cs.meta
@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: e7c6c7ce249ae9b44821e998e0331a52
\ No newline at end of file
diff --git a/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs b/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs
new file mode 100644
index 000000000..f3cf28189
--- /dev/null
+++ b/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs
@@ -0,0 +1,21 @@
+using Unity.Mathematics;
+
+namespace ProjectM.Simulation
+{
+    /// <summary>
+    /// Pure helpers for the LANTERN "light is territory" relevancy prototype (Build Spec Phase 1 step 8): an entity
+    /// outside a player's lamp radius isn't replicated to that player. The decision is a planar (XZ) distance test
+    /// so it's Burst-safe + unit-testable independent of the netcode <c>GhostRelevancy</c> set manipulation.
+    /// </summary>
+    public static class LightRelevancyMath
+    {
+        /// <summary>Prototype lamp radius (world units). Per-player dim/bright (a variable radius = the quiet/blind
+        /// trade) is the tuning follow-up; the prototype uses one fixed radius to prove the gamma test.</summary>
+        public const float LampRadiusDefault = 25f;
+
+        /// <summary>True when the ghost at <paramref name="ghostPos"/> is OUTSIDE the lamp centred on
+        /// <paramref name="playerPos"/> (planar XZ distance beyond the radius) — i.e. hidden from that player.</summary>
+        public static bool IsHidden(float3 playerPos, float3 ghostPos, float radiusSq)
+            => math.distancesq(playerPos.xz, ghostPos.xz) > radiusSq;
+    }
+}
diff --git a/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs.meta b/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs.meta
new file mode 100644
index 000000000..e3ab41f1b
--- /dev/null
+++ b/Assets/_Project/Scripts/Simulation/World/LightRelevancyMath.cs.meta
@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: e2224a38c42888d408d54961131dfff3
\ No newline at end of file
diff --git a/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs b/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs
new file mode 100644
index 000000000..2a1c3bdf5
--- /dev/null
+++ b/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs
@@ -0,0 +1,36 @@
+using NUnit.Framework;
+using ProjectM.Simulation;
+using Unity.Mathematics;
+
+namespace ProjectM.Tests
+{
+    /// <summary>
+    /// Unit tests for the LANTERN light-as-territory relevancy DECISION (LightRelevancyMath). The full per-connection
+    /// GhostRelevancy set population is netcode-world (the L3 gamma test); this pins the pure planar-distance rule.
+    /// </summary>
+    public class LightRelevancyMathTests
+    {
+        [Test]
+        public void InsideLamp_IsVisible()
+        {
+            float r = LightRelevancyMath.LampRadiusDefault;
+            Assert.IsFalse(LightRelevancyMath.IsHidden(float3.zero, new float3(r - 1f, 0f, 0f), r * r), "just inside the radius = visible");
+            Assert.IsFalse(LightRelevancyMath.IsHidden(float3.zero, float3.zero, r * r), "at the player = visible");
+        }
+
+        [Test]
+        public void OutsideLamp_IsHidden()
+        {
+            float r = LightRelevancyMath.LampRadiusDefault;
+            Assert.IsTrue(LightRelevancyMath.IsHidden(float3.zero, new float3(r + 1f, 0f, 0f), r * r), "just beyond the radius = hidden (nothing to see)");
+        }
+
+        [Test]
+        public void Planar_IgnoresY()
+        {
+            float r = 5f;
+            // Within the planar (XZ) radius but far in Y is still VISIBLE — top-down relevancy ignores height.
+            Assert.IsFalse(LightRelevancyMath.IsHidden(float3.zero, new float3(2f, 1000f, 2f), r * r), "Y is ignored (planar XZ test)");
+        }
+    }
+}
diff --git a/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs.meta b/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs.meta
new file mode 100644
index 000000000..7e8f84c68
--- /dev/null
+++ b/Assets/_Project/Tests/EditMode/LightRelevancyMathTests.cs.meta
@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: 14e936313eb9a864181228bcee28c321
\ No newline at end of file