The idea of a Hidden Warrior sickness, which is mentioned in the second part of the first book is something I discovered in a single passage from a translation of True Path of the Ninja.
I really like the idea of how this works with the concept of an illusionist who has gone so deep into tricking the world that they have now tricked themselves of this utopia that exists only within their mind, and how this could become an affliction.
This is something I will explore in later books.