Modify your Master Page to display the top level web description

1) Create a new Web Control

The web control shall retrieve the description from your top level site.

In your Visual Studio project, right click and choose Add > New Item... In the New Item dialogue select Module. Give the new module a name (SiteDescription) and click OK.
Remove the txt file.
Add a class (Add > Class...), give it a name (SiteDescription) and click Add.
The source code of your class will look something like this:

namespace NameSpace.SiteDescription
{
    /// <summary>
    /// SiteDescription class is used to displayes the description of the top level web.
    /// </summary>
    [ToolboxData("<{0}:SiteDescription runat=server></{0}:SiteDescription>")]
    public class SiteDescription : WebControl
    {
        private string _alt = "- default description of site";
        /// <summary>
        /// override so that the surrounding tag is a div insteadof a span
        /// </summary>
        protected override HtmlTextWriterTag TagKey
        {
            get { return HtmlTextWriterTag.Div; }
        }
        /// <summary>
        /// Add the description as an em tag
        /// </summary>
        /// <param name="writer"></param>
        [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust"
            )]
        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            writer.RenderBeginTag(HtmlTextWriterTag.Em);
            writer.WriteEncodedText(_alt);
            writer.RenderEndTag();
        }
        private void InitiateDescription()
        {
            try
            {
                var webApp = SPContext.Current.Site.WebApplication;
                SPWeb siteToUse = SPContext.Current.Web;
                foreach (SPSite site in webApp.Sites)
                {
                    if (site.RootWeb.Title.Equals("Title")) /* your title...*/
                    {
                        siteToUse = site.RootWeb;
                        break;
                    }
                }
                if (!String.IsNullOrEmpty(siteToUse.Description))
                {
                    _alt = siteToUse.Description;
                }
              
            }
            catch (Exception e)
            {
                /*error handling*/
            }
        }
        /// <summary>
        /// Create new web control
        /// </summary>
        public BannerDescription()
        {
            InitiateDescription();
        }
    }
}

Edit the Elements.xml file of your new Web Control:

<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Control
   Id="SiteDescription"
   Sequence="24"
   ControlClass="Namespace.SiteDescription.SiteDescription"
    ControlAssembly="Namespace, Version=1.0.0.0, Culture=neutral, PublicKeyToken=xxx">
  </Control>
</Elements>

2) Use your description web control in your master page

<NameSpace:SiteDescription ID="SiteDescriptionWebControl" runat="server"/>

Visual Studio will prompt you with the right import statements.