soul

Q: What happens to a person's soul when he dies? Also, when we say ,"We return to Allah," what does that mean?

Q: Is there any ruling that someone who committed suicide or accidental brutal death or anyone purposely killed him either ways, so their soul is roaming that place and makes hurdles to the passers and if it likes someone, it possesses the body. As this is a worldly talking that a person is been killed or died in accident or suicide...and he later possessed someones body....and troubles....so is this correct.... 

Q: I have heard from many people that our souls were already created and we had met in a place only Allah knows, long before we were born. Is it true? And what hadith and Quraan says about it?