What is the ruling on someone who commits a major sin thinking it is a minor one, and if he had known its true nature, he would not have committed it? And does he enter Hellfire if he dies after committing it, knowing that he is enchanted?

The ignorance that excuses a person is ignorance of prohibition, but ignorance of punishment does not excuse him. So, whoever knows that an act is forbidden, that is sufficient for him to incur its consequence, even if he is ignorant of the punishment entailed, whether worldly or in the hereafter. As for one who commits a sin under duress, there is no accountability for him, because Allah has overlooked for this Ummah what is done by mistake, out of forgetfulness, or under duress. In any case, whoever commits a sin should repent to Allah and seek His forgiveness, for Allah accepts repentance from His servants and pardons evil deeds, and one who repents from a sin is like one who has no sin.