Dear Sir, in the facts given and mentioned by you above, the right strategy for you should be to immediately file a suit for possession and asking for the interim stay on the ground that for over 20 years i am living peacefully having possession and sale deed in my name. 

File the suit and get stay to restrain z from interfering in your property.

Mr. X should not vacate the property under any circumstances in absence of any proof given by Mr Z showing the legal ruling which is in his favour.

Also complaint should be lodged against Mr Z for the offence of criminal intimidation under section 506 of the Indian penal code for threatening Mr X.

Also, suit for injunction can be filed against Mr Z in Civil Court in order to refrain him from disturbing the Peaceful possession of Mr X.

Yes.  Case can be filed against landlord, collaborator and the broker if the aforementioned Act was done in Collusion and connivance between them as they then you that you were the registered owner of the property and still proceeded to sell it to someone else.

1. As per the information mentioned in the present query, makes it clear that X has been harassed for no reason.
2. The period of 20 years has already closed all doors for anyone even if they had any legal right in the property.
3. Yoou should file complain against the person who is trying to enter into your portion of the property.
4. As it is not clear that whether he is saying true or not about any document been entered between with landlord or builder, and you also have no idea about it for now.
5. Please, be in possession of the property as no one can put you out of the house, it is for that man to file a suit for possession against you to prove his part.

You can make them a party.

Sir the order bought by Z needs to be seen and further challanged that on what grounds z has got order.in his favour. Further police has come meaning there by are execution order which needs to be stayed too.

See the order needs to be challanged filing a harrasment case is not way out here in case Z has got any rights same has to be contested though case of cheating and breach of trust can be filed against the builder.

You cannot be forced to vacate the flat 

2) ask Z to furnish certified copy of court order 

3) take legal proceedings to set aside court order as you are bonafide purchaser of value abd had purchased flat by regs sale deed 20 years back 

it is necessary to peruse documents of title in Z possession to advice 

2) if flat had been sold by Y to Z file case of cheating against Y and file civil suit to recoveryiur money with interest 

X have to file a suit for temporary injunction in the Court wherein X have to prove by an affidavit that her property is in danger of being damaged.

Yes a civil case can be filed but before try to get the copy of new sale deed and get it checked by an Advocate. 

If the same is not of your property than it is possible that the person is mistaken. Get the papers of get the compliant filed with police.

However u always have a good chance of restricting him by filing a case. 

Also check out your pappe that if there is no confitcon mentioned for further transfer.

The police cannot interfere in this matter without an order by the court.

You may insist on the court order and then challenge the same in the court, do not vacate, he cannot forcibly evict you without  serving a proper notice and order of the court.

You can immediately approach court and obtain a stay or injunction order restraining him from indulging in any such illegal activities other than by due process of law.

Discuss with your advocate in detail before taking any legal action on this.

Can X file a case against the Landlord, Collaborator Y and the broaker who had taken commission and got the original sale deed organised between X, Y and Z for harasement. Note pary X is a widow and a poor family.

If X is having a registered sale deed, then she can very well obtain a stay order against such atrocious people. consult an advocate and initiate proper steps immediately