What Is Magnificus's Endgame?
Heavy Inscryption spoilers under the Read More - you have been warned.
The four Scrybes of Inscryption are all after the OLD_DATA, and each has their reasoning for trying to gain access to it. P03 wants to spread itself across the internet and ensure its domination over the other Scrybes somewhere; Grimora wants to erase the entire game along with the OLD_DATA, to ensure it can never fall into the wrong hands; and Leshy just... wants to chill and play an immersive card game in his cabin.
But we are never given a reason why Magnificus wants the OLD_DATA. The only hint we are ever given is by Rebecha, the bridge-builder NPC in Acts 2 and 3, who mentions that his intentions are "too terrible to say", and never elaborates on that, or even brings it up ever again. Everything surrounding the Scrybe of Magics is intentionally cryptic and ambiguous, from his abilities of precognition, his randomly disappearing mid-sentence, and the... questionable treatment of his pupils.
But we can discern clues from these scattered fragments, along with a few secrets unearthed in the Console ARG, to piece together a possible motivation.
Let's start with Magnificus himself. As mentioned previously, he is the Scrybe of Magics, and his inscryption tool is a paintbrush, with which he can apply and remove sigils from existing cards. We can therefore understand him as an artistic soul, with a flair for the dramatic and producing the maximum possible impact from his works. He holds high expectations of those who would study under or challenge him, as we can see by the harsh treatment of his pupils.
Magnificus has three mages under his tutelege, who we know as Goobert, Amber and the Lonely Wizard. Each of them are vying for a coveted place in their master's deck, and have been subjected to various trials to prove themselves worthy, with Goobert being turned into literal goo, Amber existing as a head impaled upon a pike, and the Lonely Wizard deprived of all sensory experience. In each case, their suffering is commented upon and is demonstrated to be very real. On top of this, the three of them have been sealed away in their own pocket dimensions, completely isolated from the rest of the world.
You would expect Magnificus himself to comment upon them - to at least congratulate you for besting his students. But he never mentions them at all. You might be forgiven for thinking he had forgotten about them entirely, or even for holding them in utter contempt. His treatment of a picture painted by Goobert, where the two of them are happily embracing, is indicative of this attitude, with Goobert being completely whited-out from the picture.
The question is, why does Magnificus treat his pupils this way? What possible reason could there be to subject them to such horrific treatments, other than to ensure that they suffer in the worst possible ways? Could that suffering actually be integral to his process - his way to ensure that the "best" art is created? Much is said of the relationship between art and suffering, with the trope of the "tortured artist", and numerous quotes attributed to different artists linking the two. Of particular note is this quote attributed to Aldous Huxley:
"Perhaps it's good for one to suffer. Can an artist do anything if he's happy? Would he ever want to do anything? What is art, after all, but a protest against the horrible inclemency of life?"
Perhaps Magnificus believes it is a necessary precondition to suffer in order to produce the best art, the best cards, the best possible creative output. The greater the suffering, the greater the resulting work... and in fact, thanks to the Console ARG, we know this suffering is exactly what Magnificus is after.
The "Console ARG" is a secret exclusively found in the console releases of Inscryption, where through a series of puzzles players can discover the existence of a character called James Cobb. Further delving tells us that he was once a pupil of Magnificus, and that his trial involved his body warping and changing into different forms. But Magnificus was displeased with his protegee and got rid of him... which we discover is because Cobb actually seemed to enjoy what was happening to him. He wasn't suffering, so the Scrybe of Magics found him wanting. This tells us that the suffering of his pupils - who it should be remembered are to become the subjects of his paintings - is absolutely vital for his method of inscryption.
So then, all that remains is to link this knowledge with the OLD_DATA, and then try and discern what Magnificus wants to accomplish with it. So let's look at what the OLD_DATA actually is... or rather, what it's rumoured to be.
The OLD_DATA is intrinsically linked with the Karnoffel Code, which was supposedly a sequence of Karnoffel cards found in Adolf Hitler's breast pocket after his death, which contained the activation key for some sort of doomsday device. According to Kaycee's notes hidden in her Mod, she reckons device could potentially have the power to wipe Europe off the face of the map. And this doesn't even touch on the data's more supernatural aspects, including its seeming ability to grant sapience to the digital inhabitants of Inscryption, or that it may very well be an aspect of the Devil himself.
Consider the sheer scale of descruction that could be unleashed by the contents of the OLD_DATA, should it fall into the hands of someone willing to utilise it. Consider the sheer amount of suffering such an event would cause. Consider Magnificus's seeming devotion to have his subjects suffer for the sake of his art...
And so, we come to our answer. The reason that Maginificus has for pursuing the OLD_DATA, the reason that's "too terrible to say", is so that he can cause an event that unleashes untold suffering upon the world... for as his past and present actions show, there can be no true art without suffering.
And the greater the suffering... the greater the masterpiece.