You cannot take any precautionary measure into this before knowing what kind of trouble or torture he may unleash against you.

Therefore you may wait for the developments if any and then get into proper legal action to curb the menace with the help of law enforcing agencies.

It would be improper to suggest you on the measures to be taken without knowing the offence or the nature of offence that has been committed against you.

Besides you have not married your fiancee yet, hence in case she has any problem it is only she who has to take steps to solve the problems, though you can assist her but cannot involve directly until you are targeted or attacked by the said narcissist 

You can file a police complaint if he harrases you

There is no procedure in law for taking preventing measure. You can only react if he tries to create nuisance for you. Even at slightest trouble approach police with complaint against him. In such matters police take stringent action against the accused.

If he threatens your fiancée she can file police complaint against him for criminal intimidation under section 506 of IPC 

if he maligns her reputation she can file complaint of criminal defamation under section 500 of IPC 

If you have any evidence of that person's threat or blackmail, you may lodge a complaint with the police. Otherwise, please carry on with your married life peacefully for now.

- If you are having apprehension that he may create trouble to your relation with your fiancée, then you or she can lodge a compliant against him before the police official . 

- Further , if he try to defame you through the social media then also you can take legal action against him. 

- Section 66A of the IT Act, prohibits the sending of offensive messages though a communication device, i.e. through an online medium.

- This covers, offensive messages of a menacing character, or a message that the sender knows to be false but is sent for the purpose of ‘causing annoyance, inconvenience, danger, obstruction, insult, injury, criminal intimidation, enmity, hatred, or ill will.

- The person who booked under Section 66A could face up to 3 years of imprisonment along with a fine.

1. You have no fear about any legal action from the said person in future if he was not married to  with your fiancee.

2. Find out the extent of involvement they had for which he can get the chance to create legal trouble.

3. If their disassociation is clean and complete, you can take your relationship with her to the next step.

Yes, you can file a complaint against that guy after your marriage. Once your fiancee becomes your wife.

Dear Client,

you can lodge complaint with police if he disturbs your married life and if he is troubling your fiancee now, then immediate action will be taken by police upon filing complaint under mental harassment, physical and mental abuse.