Question: I wanted to know about the torment that sinners would be put through in their graves? I have read many hair raising accounts of this agony. 
    Answer: The punishment mentioned in the grave is specifically for people on whom the truth is totally unveiled in this world and they deliberately reject it. Such people are treated as criminals are before receiving their punishments and shown their dwelling place in Hell. This is a kind of a mental torture for him. We know from the Qur’an that the Pharaoh and his people who deliberately rejected the message of Moses (sws) are put through this torment ever since their death:

So Allah saved him from the evils that they plotted [against Him], while an evil torment encompassed the Pharaoh’s people. The Fire, they are exposed to it, morning and afternoon. And on the Day of Judgement it will be said to the angels: ‘Cast the Pharaoh’s people in a more severe torment. (40:45-6) 

    People on whom the truth is not unveiled in its ultimate form in this world, do not undergo this torment. It is evident from the Qur’an that their souls are put to sleep and they will be rewarded or punished on the Day of Judgement when they will be presented with the account of their deeds. 
    Moreover, it should be noted that this punishment does not take place in the grave because the soul is not there. It is only the physical remains of a person that are found there and which are incapable of feeling any pain or anguish. It is the soul which is tormented. It is known from the Qur’an (23:100) that souls of the dead reside in a place beyond the barrier of Barzakh, whose details we know very little of.