This is the time to see a lawyer. You have all of the information you will need.
Bring the lease and any other relevant information (the letter; the police reports, etc.)
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Based on what you have described, it appears that you have a number of potential options. The landlord has likely violated c. 186 s. 14 of the Mass General Laws, which deals with a tenant's right to the quiet enjoyment of their apartment. The landlord may also have violated ch. 93A, the Consumer Protection Law, and c. 186 s. 18, which deals with unlawful retaliation. You should look for an attorney who specializes in landlord-tenant law and who can describe the various potential options before you and what might be the likely outcomes depending on what course of action you take. Sometimes, a letter from an attorney putting the landlord on notice that they face significant liability is enough to change their behavior and potentially get you some compensation. Failing that, you could consider an application for a temporary restraining order against the landlord or you could file a civil case and ask for an injunction against what appears to be the illegal behavior of the landlord. An experienced landlord-tenant attorney should be able to discuss these various options with you and give you an understanding of the pros and cons of each approach, and their costs, in time and money. You could try talking to more than one attorney to see whose approach you prefer. Also look for good references/reviews - both in terms of quantity and quality of the reviews.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.
I agree that you should speak with an attorney.
If the landlord files eviction within 6 months of you making certain types of complaints, you will be entitled to a claim of retaliation. Assuming that you have a lease agreement, and you are not in breach of your agreement, and you are not late on rent, then you may have several good defenses to eviction.
Assuming that your landlord wants you out, you may want to use this as an opportunity to request some financial assistance in return for your promise to vacate the property.