What is the punishment for one who swears by Allah but does not fulfill his oath and does not offer expiation for it?

If a person swears an oath and then breaks it, they must offer expiation for the oath; otherwise, they are sinful. The expiation does not lapse with the passage of time. This applies if the person did not intentionally lie in their oath. However, if they intentionally swore a false oath, their oath is called "al-yamin al-ghamūs" (the immersive oath), and it is one of the gravest major sins for which there is no expiation. It is named as such because it plunges its taker into Hellfire.