This is a good rate promotion that I have taken advantage of numerous times over the years. It is valid at limited number of properties in United Kingdom and Europe, and is typically available closer to the arrival date than further out. You pay the best available rate for the first night and the second night is free. You need to use rate code IDEX1 to book this special rate, or use the link that you can find below.
This rate is eligible for Priority Club stay credit and points. Sometimes this rate, however, posts as a non-qualifying one, but a quick email (or two) to the Priority Club service center normally straightens things out.