A lot of things.

Api/Controller/ThreadController.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace RomhackPlaza\Master\Api\Controller;
+
+use XF\Api\Controller\AbstractController;
+use XF\ControllerPlugin\UndeletePlugin;
+use XF\Entity\Thread;
+use XF\Mvc\ParameterBag;
+
+class ThreadController extends AbstractController
+{
+    public function actionPost(ParameterBag $params)
+    {
+        $this->assertApiScope('thread:write');
+
+        /** @var Thread $thread */
+        $thread = $this->assertRecordExists('XF:Thread', $params->thread_id, ['FirstPost'] );
+        if( $thread->discussion_state !== 'deleted' )
+            return $this->error("This thread is not deleted.", 'not_deleted' );
+
+        if(!$thread->canUndelete($error)){
+            return $this->noPermission($error);
+        }
+
+        $thread->discussion_state = 'visible';
+        $thread->save();
+
+        return $this->apiSuccess([
+            'thread' => $thread->toApiResult()
+        ]);
+
+    }
+}