What is the ruling on believing that whoever dies suddenly from an illness has been subjected to sorcery, and that sorcerers take their soul to be their slave, and is this permissible in Islamic law?

When someone dies suddenly, their soul departs their body just like anyone else, and the soul is a matter of the unseen. The belief that the soul transfers to another (reincarnation) after death is false and a grave disbelief, and it entails denying the torment of the grave.