Landlord refusing to refund the security deposit

Hello, i am a tenant in kolkata..I have been staying in a single rented room for 4 months.I paid 1 month rent as security deposit and was told by my landlord to give a 1 month prior notice in case of moving out the property and the money will be refunded the moment i left. There was no written agreement done. The electricity in my room was supposed to be separate with a separate sub-meter. But lately it came to my notice that my landlord was secretly running many of his own appliances using my power line. So i gave a 1 month prior notice before moving out. but now they are forcing me to stay 1 extra month or they wont refund my decurity deposit. so sir/mam what should be my approach? please help my religioun is hindu(bengali)