The Book of Demons
When the students and disciples of Mount Cae eventually depart the safe haven after completing their training, they always go in pairs of two.
One of them is the brawns, specializing in numerous fighting techniques while the second is the brain of the operation, specializing in various ranged skills and arts along with the task of being the physical vessel for the 'Book of Demons'.
The 'Book of Demons' is a living entity, an artifact created from the flesh, bones, and skin of heinous and wicked humans that have led lives full of corruption and wickedness.
It takes the shape of a normal book with a thick leather cover but after being inscribed by countless various depictions of all the different types of demonic beings that walk the mortal plane, it takes on an unholy life of its own.