Added the actual logic

app/Http/Kernel.php

@@ -2,6 +2,7 @@
 
 namespace Pterodactyl\Http;
 
+use Pterodactyl\Http\Middleware\MaintenanceMiddleware;
 use Pterodactyl\Models\ApiKey;
 use Illuminate\Auth\Middleware\Authorize;
 use Illuminate\Auth\Middleware\Authenticate;
@@ -108,6 +109,7 @@ class Kernel extends HttpKernel
         'can' => Authorize::class,
         'bindings' => SubstituteBindings::class,
         'recaptcha' => VerifyReCaptcha::class,
+        'maintenance' => MaintenanceMiddleware::class,
 
         // Server specific middleware (used for authenticating access to resources)
         //

app/Http/Middleware/MaintenanceMiddleware.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Pterodactyl\Http\Middleware;
+
+use Closure;
+
+class MaintenanceMiddleware
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  \Illuminate\Http\Request  $request
+     * @param  \Closure  $next
+     * @return mixed
+     */
+    public function handle($request, Closure $next)
+    {
+        $server = $request->attributes->get('server');
+        $node = $server->node;
+
+        if ($node->maintenance) {
+            return response(view('errors.maintenance'));
+        }
+
+        return $next($request);
+    }
+}