Wizard of the dead

An employee in the Case Archives Department found a few papers related to a mysterious case, related to dozens of people who were burned and their bodies suddenly melted while they were digging in search of a huge treasure.
On the other side is a washer of the dead and his son, who has two jinn companions, as if they were about to wash the body of a great magician, before the magician shocked them by getting up from the washing stage, to begin a possessed journey full of curses and horrific phenomena.
What is the secret of this case and the employee's relationship with the magician, the dead washer, and his son?