Sale agreement gone wrong

If I had bought a house and paid for it in cash from the so called owner. After receiving the keys and staying in the house with my family for three months, the correct owner of the house appears and are threatening me and my family to be put out of the house if I do not move out. What is my legal position???

I approach you, my lawyer, as I want your advice on whether to leave the premises because the new owner has now claimed to be the lawful owner of the house.