Half an hour before Golden Mage's appearance,
Hogwarts
It was a large, empty classroom.
Professor Lupin was standing near an ancient-looking cabinet, which kept rattling with loud noises.
It was obvious that this was not one of the ordinary Boggarts.
Lupin, with Dumbledore's permission, had brought one of the fear-consuming cabinets that almost guaranteed that the Boggart within would assume a Dementor's form, especially if that fear was because of anything related to Azkaban.
"Patronus Charm?" Black asked.
Lupin nodded. "As I told Harry too," he said, pointing at Harry next to Black, "this Spell is well beyond Ordinary Wizarding Level. When executed perfectly, the Patronus Charm conjures a Patronus. Take it as an anti-Dementor… a guardian or shield between you and the Dementor."
Black listened carefully.
The Ad Infinitum alloted its full processing power to the matter, too.
Neither Kai nor Black needed to learn the Patronus Charm to fight off a Dementor. In a way, it was like a true dark magician learning a holy spell. As if that wasn't ironic enough, things became even more ridiculous when one remembered that Kai not only had a Dementor Blood Essence, but he literally had a full legion of Dementors… created by him.
So, no.
Black wasn't bringing his full attention just to fight off a Dementor.
He was doing it to learn the Patronus Charm itself; truly.
The Ancient Magical Script allowed one to replicate all Spells and even create one's own Spells, if one had an AI brain to shorten the decades and centuries needed to do so.
It was all about a matter of meaningful arrangement of the Script's glyphs, which could result in a desired effect once carved, whether on paper, air, or any other object.
However, there was a reason that the System had only accepted Black's Band of Light, which he had created with Medea's help, as a Spell, and not the flashy display of magic he had shown as a Hogwarts student until now.
The keyword here was arrangement.
No matter which Spell one deciphered or created using Ancient Magical Script, it was unique — just like Shirou's Trace On, Medea's Rain of Light, etc.
Kai could decipher Expelliarmus and Avada Kedavra spells and create his own versions, but they would never be the same as the true Disarming Charm and the Killing Curse.
The same was true for the Patronus Charm, too.
Kai meant to keep the Dementor Blood Essence for a long time, as it awarded him Breath Points out of the Set Quota. However, that also created a weakness, which the likes of Herpo the Foul knew now. It would be safe to say that the matter of Dementors being a part of his power must have already gotten out, reaching the Primordial Tower.
Lucky Intuitions had already warned him that there were high chances of a powerful Contestant using the Patronus Charm on him in the future.
Even if he kept the Dementor Blood Essence deactivated, it might still end up affecting him.
And, at his level, even a momentary unresponsiveness could decide life or death.
So, Kai must learn to cast the Patronus Charm — to study it — firsthand.
Things were already happening as he had planned until this point, with only minor divergences.
"The Patronus is kind of a positive force," Lupin said. "A projection of what a Dementor feeds upon — Hope, Happiness, Will to Survive. But it cannot feel despair. Not as humans can, making it impervious to the Dementors. To cast it, you need an incantation, which will work only if you are concentrating, with all your might, on a single, very happy memory."
As Lupin began teaching Black how to clearly pronounce Expecto Patronum, the Ad Infinitum had already begun conjuring a set of the strongest, happiest memories.
The plan was not only simple but also infallible.
Once the set of happy memories, thousands in number, was generated, the Ad Infinitum would begin using the three Contrasts of Facelessness, letting Black believe that these memories belonged to him from all his heart, even if they were fake.
Whichever memory brought out the strongest reaction from the entire mindscape when facing the Dementor, the AI would keep it, discarding the rest.
The only thing that could affect Black's success was the Wand… because there wasn't a Wand.
He was using a fake, not a proper Wand turned into a Item.
Instead, Black would be using Ancient Magical Script to decipher the Patronus Charm's structure and cast it himself.
He couldn't have done it before, but presently, he had already met all the conditions.
Professor Lupin was himself teaching him the Patronus Charm, just like Medea had taught him the Rain of Light Spell. It would make sure that the Patronus would keep growing along with him.
He had also met a hidden condition, which Meg had told him about, stating that Lupin must not teach the Spell forcefully.
Everything was as Black intended it to be.
"Are you ready to try it on a Dementor?" Lupin asked. "Remember, you must focus on the happiest memory you have. The happier the better."
Black nodded as Harry walked slightly away from him after patting his shoulder.
Lupin flicked his wand, and the cabinet opened with a bang.
Lamps flickered, and a chill took over the entire classroom instantly.
A tall Dementor slithered out of the cabinet, its tattered black cloak billowing behind it.
Black raised his Wand, letting the Dementor affect his emotions, and the Ad Infinitum began scrolling through the memories with unimaginable speed.
Tiny, bluish-silvery runes danced in Black's surroundings, stunning both Harry and Lupin.
In a fraction of a second, the AI found the memory that brought the strongest reaction inside and outside.
It was a conjured memory of Alan and Edith Stormborn showering love on a newborn Kai, all the yellow crystals in Black's mindscape twinkling with the brightest flash in response.
Even the Dementor outside suddenly shuddered, immediately sucking in deeply as if the emotions it had been craving for had just erupted out of the boy.
"Expecto—" Black said, the runes around him glowing brighter than ever. "—Patronum!"
Harry's eyes popped out.
A silvery mist escaped from the tip of Black's wand, becoming a disc in front of him.
Then, just before Harry and Lupin could fully smile, the silver mist thinned out as if it had never been there.
And the Dementor fell upon Black like a hungry beast, making him pass out.