@Ceiye, their is a better plan :
- kill the killer (his name is pillow san i think),
- then disguise his corpse as the great mage,
- then disguise himself or the great mage as the killer.
-> result :
- save the great mage,
- don't change the history because everyone would still believe that the great mage is dead
- take revenge on the killer
- spy undercover on the organization that tried to kill the great mage.