What are a tenant's rights against their landlord for allowing an unruly neighbor to reside on the premises?

Question Details:

Renting basement unit for 15 years. Year ago new tenant moved in above me (loud music, parties, obnoxious person). Many nights we had to leave and come back later because of the noise. Myself as well as other tenants went to our landlady for help. She spoke with him many times about the noise complaints, but it was of little use. Police have been to his door several times because of noise, and there have been physical threats. I believe my unit is illegal. We are moving but want to take legal action against the landlady for the uncomfortable living conditions that we have put up with.

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