Regardless of how old a civilization is, generical hallucinations can still be categorized as a kind of perceptual disturbance. Even in Ancient Greece, certain individuals (Hippocrates to name one) still can interpret hallucinations as dysfunction of brain.
Therefore, creating illusions that can be logically accepted with realistic reasoning is important. What I needed was an "entity", a representative being that could bridge between the contents of my head and the reality that Liam believed in. This would not only help in the process of body control, but also become a practical tool for managing data and information that I could not refuse; imagine the folder and search features were gone, then one would have to search for documents manually.
Out of all the options I had after making several narrowings, I ended up with three; the others were either too hard to sustain or too weird, and would ironically create more problems down the road. These three while not perfect, were at least within limits that I could manage.
The first option was to establish a narrative of a "Guardian Soul", a hallucinatory appearance of something that appeared in a physical form. I could create a small animal to reduce vigilance, or an old man figure of warmth that speaks gently. It's an approach that's easy for children to digest, and it also makes sense in a world where religion monopolizes everything. However, this approach has one fundamental flaw: consistency. To maintain this illusion I need to be able to create complex patterns of behavior and response, which is not something I can do constantly, especially with the limited control over Liam's brain. If this character changes without a reasonable reason, then Liam might get suspicious. I didn't want to risk everything just because I forgot how the "old man" sounded or spoke before; the old me probably wouldn't forget it, but the me who was now bombarded with various mortar data was a different scenario.
The second option is "Divine Voice", I can create a simple form of auditory hallucination; that is, a voice that seems to come from a great entity. This fits perfectly into the setting of Liam's world which is still within the conventional belief system, a child like Liam would be more receptive to the idea that there is a "God" who speaks directly to him: giving instructions, even orders, with every prohibition; fanaticism can be formed from here, and every action can be justified as the "path of righteousness". But the main risk comes not from within, but from without; if Liam were to tell others about this "God", it could prove fatal. The world is most likely not a tolerant place for religious deviations; Liam could be labeled a heretic, possessed, or even worse be subjected to torture by the religious right. Moreover, the divine voice's communication style tended to be rhetorical and full of symbolism; making it unsuitable for conveying the quick and technical commands needed in the manipulation process.
The third option was the scheme I eventually chose: RPG system. It was a combination of various types of hallucinations put together within a functional narrative framework; although it sounded complex, it was a much lighter and more manageable approach. I simply needed to layout the information that I was going to display in the form of a visual. The information itself doesn't always have to be accurate; it's not about the authenticity, but feeling of dominance and consistency between sections. The system presents the illusion of being more interactive and gamified, an approach that has been proven to be highly effective at building motivation and addiction; 3.3 billion people have already proven it. Challenges, rewards, leveling up, and achievements are psychological mechanisms that have been tested millions of times. And... It, always works.
In terms of management, it's much more practical. As a former retailer who used to organize various documents; from the seemingly legal to the strangely absurd. The RPG system provided a mechanism for managing data processing software: input, process, output, and full control over the flow. I could compose, edit, or delete quests as needed; without the need to build a complicated interpersonal narrative like in the "Guardian Soul" scheme.
And most importantly, if Liam one day leaked the existence of this system, the impact would not be as severe as claiming to hear the voice of God; at most, he would be considered weird or perhaps labeled as lunatic. There would most likely be no threats from religious institutions or witch hunters, as was common with rebels of religious dogma in medieval time. The illusion of the RPG system can still be digested as an absurd children's fantasy, not an affront to doctrine or scripture.
Of course, there are still flaws. The RPG system contains many modern terms such as: levels, inventory, statistics; things that would be foreign to someone from the medieval world. But this problem is not a big hurdle. Just like I could understand some words of this world because I was now living in Liam's body, so was the other way around; I could inject understanding slowly, as if Liam already knew the meaning of those terms from the start.
The issue of trust is not a big one. Liam's natural curiosity will provoke repeated interactions, and from that will grow the seeds of trust. If it was strong enough, this trust could grow into fanaticism; and when fanaticism was formed, then the control over Liam's body had already reached 80%.
I didn't expect that the RPG information from the case of money laundering through RMT video games could be used like this; this case used to interest me, making me explore various fantasy worlds. And if I were to be honest, I have no interest in playing video games or reading fictional stories; however, I don't have any confidence issues if I have to argue about it.