I am sorry to learn of this situation.
Unfortunately these "land purchase contracts" (or "carrying paper" or "owner financed") transactions are inherently risky - both sides (both buyer and seller) seem to wind up with a lot of legal problems.
As a bona fide buyer, with actual documentation to support your purchase, you should hire a local civil litigation attorney (right away). (I fully understand that customers do not come to this site to be told "go hire a lawyer" but I want to tell you why I give this advice for your specific question here).
Your seller is engaged in fraud, and very likely has some mortgages out on the property that need to be dealt with - so you not only have to fend off the seller, but also any of his creditors associated with the property as well.
You do have rights (you bought the property), but you must work swiftly to protect yourself, and hiring a lawyer now will reduce your overall costs of having to defend the action.
You can find local property law or civil litigation attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).