diff --git a/src/common/rpg/components/effect.ts b/src/common/rpg/components/effect.ts
new file mode 100644
index 0000000..5134518
--- /dev/null
+++ b/src/common/rpg/components/effect.ts
@@ -0,0 +1,83 @@
+import { component } from "../core/registry";
+import { Component, type EvalContext } from "../core/world";
+import { evaluateCondition } from "../utils/conditions";
+import { action, variable } from "../utils/decorators";
+import { Stat } from "./stat";
+
+@component
+export class Effect extends Component<{
+    targetStat: string;   // component key, e.g. 'health'
+    targetField: 'value' | 'max' | 'min';
+    delta: number;
+    duration: number | null;   // null = permanent until removed
+    remaining: number | null;  // countdown in seconds; null for condition-based/permanent
+    condition: string | null;  // keep effect while true; remove when it becomes false
+}> {
+    /** True while the effect's delta is applied to the target stat. */
+    @variable('.') active: boolean = false;
+
+    constructor(
+        targetStat: string,
+        delta: number,
+        targetField?: 'value' | 'max' | 'min',
+        duration?: number,
+        condition?: string,
+    ) {
+        super({
+            targetStat,
+            targetField: targetField ?? 'value',
+            delta,
+            duration: duration ?? null,
+            remaining: duration ?? null,
+            condition: condition ?? null,
+        });
+    }
+
+    override onAdd(): void {
+        const stat = this.entity.get(Stat, this.state.targetStat);
+        if (stat) {
+            stat.applyModifier(this.state.delta, this.state.targetField);
+        }
+        this.active = true;
+    }
+
+    override onRemove(): void {
+        const stat = this.entity.get(Stat, this.state.targetStat);
+        if (stat) {
+            stat.removeModifier(this.state.delta, this.state.targetField);
+        }
+        this.active = false;
+    }
+
+    @action
+    reset(duration?: number): void {
+        if (duration != null) {
+            this.state.duration = duration;
+        }
+        this.state.remaining = this.state.duration;
+        this.active = true;
+    }
+
+    /** Mark as expired. EffectSystem will remove the component, which reverses the delta. */
+    @action
+    clear(): void {
+        this.state.remaining = 0;
+        this.active = false;
+        this.emit('expired');
+    }
+
+    update(dt: number, ctx?: EvalContext): void {
+        if (this.state.remaining != null) {
+            this.state.remaining -= dt;
+            if (this.state.remaining <= 0) {
+                this.state.remaining = 0;
+                this.clear();
+            }
+        } else if (this.state.condition != null) {
+            if (!evaluateCondition(this.state.condition, ctx ?? this.context)) {
+                this.clear();
+            }
+        }
+        // permanent effect (no duration, no condition): nothing to do
+    }
+}
diff --git a/src/common/rpg/components/stat.ts b/src/common/rpg/components/stat.ts
index f402ab3..2fe630f 100644
--- a/src/common/rpg/components/stat.ts
+++ b/src/common/rpg/components/stat.ts
@@ -3,7 +3,8 @@ import { Component } from "../core/world";
 import { component } from "../core/registry";
 
 interface StatState {
-    value: number;
+    base: number;
+    modifierSums: { value: number; max: number; min: number };
     max: number | undefined;
     min: number | undefined;
 }
@@ -11,47 +12,67 @@ interface StatState {
 @component
 export class Stat extends Component<StatState> {
     constructor(value: number, max?: number, min?: number) {
-        super({ value, max, min });
+        super({ base: value, modifierSums: { value: 0, max: 0, min: 0 }, max, min });
     }
 
-    @variable('.') get value(): number { return this.state.value; }
-    @variable get max(): number | undefined { return this.state.max; }
-    @variable get min(): number | undefined { return this.state.min; }
+    @variable('.') get value(): number {
+        const effMin = this.min;
+        const effMax = this.max;
+        let v = this.state.base + this.state.modifierSums.value;
+        if (effMin != null) v = Math.max(effMin, v);
+        if (effMax != null) v = Math.min(v, effMax);
+        return v;
+    }
+
+    @variable get base(): number { return this.state.base; }
+    @variable get max(): number | undefined {
+        return this.state.max != null ? this.state.max + this.state.modifierSums.max : undefined;
+    }
+    @variable get min(): number | undefined {
+        return this.state.min != null ? this.state.min + this.state.modifierSums.min : undefined;
+    }
 
     @action
     update(amount: number) {
-        this.set(this.state.value + amount);
+        this.set(this.state.base + amount);
     }
 
     @action
     set(value: number) {
-        const prev = this.state.value;
-        this.state.value = value;
-        if (this.state.min != null) {
-            this.state.value = Math.max(this.state.min, this.state.value);
-        }
-        if (this.state.max != null) {
-            this.state.value = Math.min(this.state.value, this.state.max);
-        }
-        if (prev !== this.state.value) {
-            this.emit('set', { prev, value: this.state.value });
+        const prev = this.value;
+        this.state.base = value;
+        const next = this.value;
+        if (prev !== next) {
+            this.emit('set', { prev, value: next });
         }
     }
 
-    get current(): number {
-        return this.state.value;
+    applyModifier(delta: number, field: 'value' | 'max' | 'min' = 'value'): void {
+        const prev = this.value;
+        this.state.modifierSums[field] += delta;
+        const next = this.value;
+        if (prev !== next) this.emit('set', { prev, value: next });
     }
+
+    removeModifier(delta: number, field: 'value' | 'max' | 'min' = 'value'): void {
+        this.applyModifier(-delta, field);
+    }
+
+    get current(): number { return this.value; }
 }
 
 @component
 export class Health extends Stat {
-    constructor(value: number, max?: number, min = 0) {
-        super(value, max, min);
-    }
-
     @action
     kill() {
         this.set(0);
         this.emit('killed');
     }
 }
+
+@component
+export class Defense extends Stat { }
+
+@component
+export class Damage extends Stat { }
+
diff --git a/src/common/rpg/systems/effectSystem.ts b/src/common/rpg/systems/effectSystem.ts
new file mode 100644
index 0000000..67c6918
--- /dev/null
+++ b/src/common/rpg/systems/effectSystem.ts
@@ -0,0 +1,19 @@
+import { Effect } from "../components/effect";
+import { System, type Entity, type World } from "../core/world";
+
+export class EffectSystem extends System {
+    override update(world: World, dt: number): void {
+        const expired: [Entity, string][] = [];
+
+        for (const [entity, key, effect] of world.query(Effect)) {
+            effect.update(dt, entity.context);
+            if (!effect.active) {
+                expired.push([entity, key]);
+            }
+        }
+
+        for (const [entity, key] of expired) {
+            entity.remove(key);
+        }
+    }
+}