I moved into a one bedroom apartment on the 9th of May, 2016, after full payment had been made to the landlady, her agent, and the 'statutory' drink money house owners in Port Harcourt will rather die than let go off. Thereafter, I requested and insisted to get my rent receipt which my landlady asked me to come get a week later.

On the said day, I went to her house and got the receipt. Set to leave, she told me to hold on, and proceeded to produce a typed document mandating me to pay N30,000 as Caution Fee which she claimed was refundable at the expiration of my tenancy where I chose not to renew. I was aghast, and calmly asked her why she didn't mention it earlier in the course of the negotiations. She was mum and I made it clear I wasn't paying. She left it at that.

Now she has resorted to sending me threat messages, which I have reported to the Police. She called me today and informed me that she will return my money to the Police and that I should pack out within two weeks.

That I cannot do. I want to know my rights as a tenant with particular reference to this case. Because I intend to sue if need be.

Ehn, I would not want to bore you with technical details but you have a valid tenancy contract. As long as you never agreed to pay any caution fees as one of the terms of the contract , you are good to go.
Still, your landlord still has the right to evict you but there are laid down procedures for that determined by the nature of tenancy. You can always sue for breach of implied covenants like quiet enjoyment and damages for breach of contract and stuff like that but it tends to be theoretical and an academic exercise. Going to court in Nigeria can be a tedious exercise.
In your shoes, I would have a lawyer write up a heavily worded "threat" to sue to the landlady for harassment, stating of course that she cannot enforce the caution fees on you, any eviction must follow the procedure provided under the law blah blah blah
Then again, you can just leave on demand of a month notice (your safety may be on the line if you stay) and then set up date where you met and she pays all the monies including the agent fees and drinking whatever to YOU not the police and make sure you never walk in alone. You would always need a witness/backup.

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