locustandi

How can you be witness without knowing the contents of the documents.You bear the witness to the document once you signed the document, even you cannot claim after a long time that you were forced to signed the document ;

When you are not the beneficiary, you can not claim any rights by being a witness to that document.  You may issue a legal notice to those parties who are involved in that execution, denying of the contents and stating the circumstances under which you have signed as a witness.

I agree with Shri G. L. N. Prasad only partly.

I agree a Witness is not a beneficiary and therefore can't challenge the Deed.

The Witness can't send a Legal Notice.    A Witness is not supposed to know the contents of any document.    He's supposed to know the persons who executed the document.