The so called blackmails holding the obscene photos are illegal acts.
Let him be rich and powerful, it will not give him licence to spoil your married life. Moreover it is more than 12 years after your marriage now so there is no reason to fear about his threats anymore.
Further, due to his threats you have already surrendered to him once earlier, if you indicate that you still afraid of him then he may take advantage at this stage too. Just ignore him and his calls, infact your decision to quit their relationship is a better decision. Nobody can force relationship on anyone, if there is any pressure, you may lodge a criminal complaint against him and see how he reacts. With your husband's support, you may ensure that a FIR is registered after which he may realise the seriousness and may avoid even talking about you.