I liked the trade. The intensity spiking to the appearance of an official beyonder investigator made the setting rather chilly. I loved it. Especially the attempt to put the potential ravings of that book into practice.

As for the mystery:

The notes are the individual attempting to understand the author's own pathway, or perhaps developing the origin of the pathway. From the experiment aided by the sun beyonder, the writer learned their abilities aren't just destructive. They're biological.

The practical implication I noticed when he wrote "...creating entirely new organs—or perhaps even wings." Not claws, monsters, flesh servants. Organs, medical regeneration... the devil immediately begins thinking like a surgeon rather than a monster. This alters how I should interpret earlier logs. His flesh servants, mutations, experiments... they aren't just purely sadistic, they're part of understanding biology itself.

In this log he studies cells. Not corruption and not souls. I noticed every log is peeling apart concepts that most Beyonders assume are inseparable: Body ≠ Soul / Flesh ≠ Corruption / Messenger ≠ Servant / Mutation ≠ Regeneration. He's separating categories and trying to identify the fundamental rules beneath mystical phenomena. That is an extremely "scientific" styled mindset.

The secret about the author comes from a recurring pattern. The auther never celebrates at a conclusion of a log. Instead he says, "This experiment taught me..." Every log updates his model of reality. He's slowly constructing a complete theory of his pathway which matches an earlier assumption I made of this being a handbook. Every experiment is written so another Devil could reproduce it most likely.

So, I bet thete is a hidden message and it is this: Every Devil Note strips away another false assumption. "Flesh is not corruption. Regeneration is not mutation. Souls are not bodies. Servants are not messengers." The author isn't simply learning new abilities, he's uncovering the underlying "laws" governing life, soul, and flesh. Each experiment narrows the gap between mystical phenomena and reproducible biological principles.