How to proceed against a life threatening warning

Hi I was in relationship with a woman for two years. We got married with mutual consent an year ago. We didn't get the marriage registered. She recently got a job in an MNC few months ago. She started cheating on me with a colleague. I found out and asked her stop. She said she did. But as she is in a different city, I was lied to and she continued with her extramarital relationship. I later came to know and questioned her. She started telling people that she never loved me and I was the one who forced her to love. She is denying the marriage. I do not have a proof of marriage but I do have chats and conversations proving our relationship. I went to her father to tell him about this. He was in another tension and I wasn't able to tell him everything and ofcourse he wasn't interested in listening either. But he got the jist of what I wanted to tell. Instead of working on what needs to be done, he threatened me not to tell anyone anything about this and he will kill me otherwise. He also said that he was an ex naxalite. Please advice on how to proceed on this. Regards and thanks in advance