Add an ItemHandler class and prevent players from having more than 1 aspect.

module/documents/Item/Aspect.mjs

@@ -0,0 +1,17 @@
+/** @this {Item} */
+async function _preCreate(data, options, user) {
+	if (this.parent.itemTypes.aspect.length > 0) {
+		ui.notifications.error(
+			game.i18n.format(
+				`dotdungeon.notification.error.aspect-limit-reached`,
+				{ limit: 1 }
+			),
+			{ console: false }
+		);
+		return false;
+	};
+};
+
+export default {
+	_preCreate,
+};

module/documents/Item/Handler.mjs

@@ -0,0 +1,21 @@
+import AspectItem from "./Aspect.mjs";
+
+export class ItemHandler extends Item {
+	itemTypes = {
+		aspect: AspectItem,
+	};
+
+	constructor(data, ctx) {
+		super(data, ctx);
+	};
+
+	/** @type {class|undefined} */
+	get fn() {
+		return this.itemTypes[this.type];
+	};
+
+	async _preCreate(...args) {
+		if (!this.fn?._preCreate) return;
+		return this.fn?._preCreate.bind(this)(...args);
+	};
+};