Builder is unfair in Timeliness and Quality

I have booked a Flat on 3 July 2013 and got the Sales and Construction agreement dated 10 Aug 2013. The builder has mentioned 24 months + 6 months grace period . The total 30 month is going to end on 12 Feb 2016. On 22 Nov 2015 , I have been to visit the Flat to know about the construction status. The Builder has used inferior quality products and construction quality is not satisfactory. Still so much construction work is pending. There are 950 flats available in the Apartment. I have got support from 60 Flat owners who is also having similar concerns. I have collected all the concerns from the Flat owners and written a letter to the Chairman& Managing Directors of Builder. I have sent the letter through Registered Post on 8 Dec 2015 , the builder received the letter on 9 Dec 2015. On 11 Dec 2015 , I have sent an email with letter in .doc and .pdf format as attachment to their corporate email Id keeping all the 60 Flat owners email Id in CC. On 14 Dec 2015 ,the builder has called all the flat owners through phone(except me) and threatening them by saying that the builder will cancel their Flat. The Builder has also informed them that they has already cancelled my Flat. The Builder wants to separate the Group. Already 6 owners withdrawn their support. The Builder has declined the Flat owners request when the Flat owners told the builder to send a written communication regarding the Flat cancellation. The builder is seeking apology letter from the Flat owners supported in the Letter. On 21 Dec 2015, I have received a letter from the Advocate of the Builder, that letter has mentioned that "I am conspiring against the builder by getting support from their competitors. The letter also threatens me that I have to pay compensation of Rs 1 crore to them if any of the flat owners cancelling their flats. The Builder has clearly mentioned that He will not answer any of the questions regarding Timeline and Quality" .