Initial

module/data/activeEffect/_module.mjs

@@ -1,11 +1,13 @@
 import BaseEffect from './baseEffect.mjs';
 import BeastformEffect from './beastformEffect.mjs';
 import HordeEffect from './hordeEffect.mjs';
+import ArmorEffect from './armorEffect.mjs';
 
-export { BaseEffect, BeastformEffect, HordeEffect };
+export { BaseEffect, BeastformEffect, HordeEffect, ArmorEffect };
 
 export const config = {
     base: BaseEffect,
     beastform: BeastformEffect,
-    horde: HordeEffect
+    horde: HordeEffect,
+    armor: ArmorEffect
 };

module/data/activeEffect/armorEffect.mjs

@@ -0,0 +1,64 @@
+export default class ArmorEffect extends foundry.data.ActiveEffectTypeDataModel {
+    static defineSchema() {
+        const fields = foundry.data.fields;
+
+        return {
+            ...super.defineSchema(),
+            changes: new fields.ArrayField(
+                new fields.SchemaField({
+                    type: new fields.StringField({
+                        required: true,
+                        blank: false,
+                        choices: CONFIG.DH.GENERAL.activeEffectModes,
+                        initial: CONFIG.DH.GENERAL.activeEffectModes.add.id,
+                        validate: ArmorEffect.#validateType
+                    }),
+                    phase: new fields.StringField({ required: true, blank: false, initial: 'initial' }),
+                    priority: new fields.NumberField(),
+                    marked: new fields.NumberField({
+                        required: true,
+                        integer: true,
+                        initial: 0,
+                        min: 0,
+                        label: 'DAGGERHEART.GENERAL.value'
+                    }),
+                    max: new fields.NumberField({
+                        required: true,
+                        integer: true,
+                        initial: 1,
+                        min: 1,
+                        label: 'DAGGERHEART.GENERAL.max'
+                    })
+                })
+            )
+        };
+    }
+
+    static applyChangeField(model, change, field) {
+        return [model, change, field];
+    }
+
+    prepareBaseData() {
+        for (const change of this.changes) {
+            change.key = 'system.armorTest';
+            change.value = Math.max(change.max - change.marked, change.max);
+        }
+    }
+
+    /**
+     * Validate that an {@link EffectChangeData#type} string is well-formed.
+     * @param {string} type The string to be validated
+     * @returns {true}
+     * @throws {Error} An error if the type string is malformed
+     */
+    static #validateType(type) {
+        if (type.length < 3) throw new Error('must be at least three characters long');
+        if (!/^custom\.-?\d+$/.test(type) && !type.split('.').every(s => /^[a-z0-9]+$/i.test(s))) {
+            throw new Error(
+                'A change type must either be a sequence of dot-delimited, alpha-numeric substrings or of the form' +
+                    ' "custom.{number}"'
+            );
+        }
+        return true;
+    }
+}