Builder delaying project handover, demanding additional ₹5 lakh for registration?

I booked a flat in a housing project in Pune 3 years ago, which was supposed to be handed over last month. Now, the builder is delaying the handover and demanding an extra ₹5 lakh for registration, claiming increased costs. This wasn't mentioned in our original agreement. I feel pressured as my family is waiting to move in. What legal action can I take against the builder to ensure the handover and protect myself from this extortion?