Give this event code a try...

Code:
Private Sub Worksheet_Change(ByVal Target As Range)
  If Not Intersect(Target, Columns("C:K")) Is Nothing Then
    If Cells(Target.Row, "B") = "" And Target.Value <> "" Then
      MsgBox "The description is blank... you must have a description before you can fill in any tasks!"
      Application.EnableEvents = False
      Application.Undo
      Application.EnableEvents = True
    End If
  End If
End Sub
HOW TO INSTALL Event Code
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If you are new to event code procedures, they are easy to install. To install it, right-click the name tab at the bottom of the worksheet that is to have the functionality to be provided by the event code and select "View Code" from the popup menu that appears. This will open up the code window for that worksheet. Copy/Paste the event code into that code window. That's it... the code will now operate automatically when its particular event procedure is raised by an action you take on the worksheet itself.