Is expiation (kaffarah) obligatory for one who swears by Allah the Almighty that there is no torment of the grave? And what are the consequences?

The torment and bliss of the grave are established by the Quran and the continuous Sunnah (definition) (Mutawatir), and this is the belief of Ahl al-Sunnah wal-Jama'ah (the People of the Sunnah and the Community). Delving into religious matters without knowledge is a great sin, for Allah the Exalted has forbidden speaking about Him without knowledge concerning His Names, Attributes, Actions, Religion, and Law.

As for the expiation for an oath, there is no expiation for an oath sworn concerning a past event, whether the swearer was lying or believed himself to be truthful. Likewise, there is no expiation for an oath sworn concerning a future event where the swearer believed himself to be truthful, but it turned out otherwise, or concerning someone else whom he thought would obey him but did not; all these are considered "laghw" (idle or vain) oaths.