Global.asax.cs

protected void Application_Error(object sender, EventArgs e)
{
    // get the last error
    HttpException err = Server.GetLastError() as HttpException;

    Server.ClearError();

    // try to skip using any other custom error settings
    Response.TrySkipIisCustomErrors = true;

    // we're on!
    if (err != null)
    {
        // get the language from the visitor's browser settings
        string lang = (Request.UserLanguages ?? Enumerable.Empty<string>()).FirstOrDefault();
        var culture = CultureInfo.GetCultureInfo(lang);

        // set the culture to match the browser language
        Thread.CurrentThread.CurrentUICulture = culture;

        PageManager pageManager = PageManager.GetManager();

        PageNode pNode = null;

        // this is where we would put a manager to do some fancy lookups based on the error codes & user selections
        switch (err.GetHttpCode())
        {
            case 404:
                pNode = pageManager.GetPageNodes().Where(pN => pN.Title == "404").FirstOrDefault();
                break;
            case 500:
            default:
                pNode = pageManager.GetPageNodes().Where(pN => pN.Title == "Generic").FirstOrDefault();
                break;
        }

        // grab the URL of the error page we're going to display to the visitor
        string url = pNode.GetFullUrl(culture, true);
        url = UrlPath.ResolveUrl(url, true, true);

        // make a web request to the URL and grab the entire HTML of the page
        WebRequest request = WebRequest.Create(url);
        WebResponse response = request.GetResponse();
        Stream data = response.GetResponseStream();
        string html = String.Empty;
        using (StreamReader sr = new StreamReader(data))
        {
            html = sr.ReadToEnd();
        }

        // send back the error page and correct status code
        Response.Write(html);
        Response.StatusCode = err.GetHttpCode();
    }
}

web.config

<customErrors mode="On">
  <error statusCode="404" redirect="~/8ec96125-02ce-47e2-a545-0eba4192dc7d" />
</customErrors>

<httpErrors errorMode="Custom">
  <remove statusCode="404" subStatusCode="-1" />
  <error statusCode="404" prefixLanguageFilePath="" path="/CustomErrorPages/8ec96125-02ce-47e2-a545-0eba4192dc7d.aspx" responseMode="ExecuteURL" />
</httpErrors>