Landlord threatening to tell my parents about my relationship. Remedy?

I am undergraduate student in my final year of my study, me and my girlfriend, we are in a live in relationship. The landlord was well aware of our situation that we posses no legal bond and neither our parents know about our situation. Now when we had to change our flat, the landlord is threatening me with not giving me my security money and also blackmailing me that he will tell my parents about it. As a student I am pretty scared of both how it will affect my career and my family. It came to a point that he came to my new flat to search for me and warn me. Kindly help.