Not declared rental income for the last 10 years, being threatened to report me to income tax

I am stuck in a difficult situation. Please give me your brutal honest opinion on my situation. Here is my brief story of how I got into this situation. I got an inheritance of 10 flats in an upscale society in Noida in 2013. Once the flats were handed over to me, I rented out all 10 flats. I received rent for all those 10 flats in cash. I never declared this income. In total I received rent of around INR 4 crore for 10 years in cash. Recently a former tenant has figured out that I am renting all the flats and taking rent in cash. And this can be traced with log books of the Resident welfare association where Tenants have paid maintenance and also electricity bills give a clue that flats were indeed rented out. This tenant wants to report me to income tax. I have sold all the flats last year and paid capital gains tax and bought new property. What are the implications if the former tenant reports me to IT? I have no other source of income to show. Resident Welfare association only show maintenance paid by the tenant but not the rent. How will the IT department act on me? How will they calculate rent and fines and penalties for those 10 years? What consequences should I be prepared for? I feel like I should come clean to IT but I am afraid. Please share your opinion without any filter of what I face once IT decides to act on me. A rough calculation of how much I have to pay now including all the fines and penalties would be really helpful.