Do i need to submit my educational documents in a company ?

Hello Sir/ Madam, I have been working for a company in Hyderabad. At the time of joining my company asked for 2 years contract(BOND), they took my educational certificates . I have been working for that company for 14 months. I have to attend VISA interview for higher studies and asked them to return my certificates, After undergoing so much stress and with so much struggle, I finally got my certificates from them. They forced me to sign the letter that I would return my certificates to them immediately after VISA processing. They threatened me that they will not give me certificates unless I sign the letter. They have been blackmailing me that there is a bond in place due to training offered to me and they were asking me to come back and surrender my certificates. Sir, Can some one help on this please.... If they approach me by sending a lawyer notice(they meant to say they are proceeding legally.), what should i do? Can i proceed further? Would there be any chance i could defend myself? Please help me.