Cassian arrives at Hogwarts to take over the History of Magic post from the ghost Professor Binns, instantly drawing the ire of traditionalists like Severus Snape, who view him as a coddled, near-Squib nepotism hire. Rather than relying on dry lectures and dusty dates, Cassian completely upends the curriculum by utilizing his newly unlocked spell, Lumos Spectaculum, to project vivid, cinematic illusions of historical events directly onto the classroom walls. He presents a gritty, unvarnished look at legends like Merlin and Morgana Le Fay, forcing the students to question the biased narratives of the victor-written Ministry textbooks. To further test his system, he ends his lessons with practical spellwork, guiding the students to deeply comprehend basic spells like Incendio and Spongify through their historical and scientific contexts. The results are immediate and spectacular, as the children's spell mastery skyrockets, which in turn elevates Cassian's own magical interface to maximum level and unlocks powerful ancient variants like Cinis Amissa and Pyroclave. This unprecedented teaching method draws the attention of the school governors, including a smug Lucius Malfoy, who attempts to intimidate Cassian during an official inspection. Cassian effortlessly slaps down the pureblood elite by delivering a blistering lecture on the historical incompetence of corrupt advisors, leaving Lucius seething in silence. His unorthodox methods spark a quiet revolution among the houses, as even the notoriously stubborn Slytherins and Gryffindors begin to think outside the rigid boxes of standard wandwork. He challenges them with creative problems, like bypassing an age line with simple Muggle physics rather than complex, volatile potions, earning him the fierce loyalty of students like the Weasley twins and Nymphadora Tonks. Yet, just as Cassian consolidates his academic authority and begins to enjoy his newfound prestige, a dark shadow falls over the castle when the Defence Against the Dark Arts professor, Helena Mulford, is found brutally cursed on the library floor, signaling that the legendary curse on the post is far more active, ancient, and lethal than anyone dared to believe, leaving the entire faculty in a state of quiet panic.

During the summer break, Cassian escapes the stifling politics of the Rosier manor by joining Bathsheda Babbling on an international archaeological expedition to a newly discovered pre-Neolithic rune site in Norway. The dig site, nestled in a frigid fjord near Trondheim, is thick with an ancient, oppressive magic that defies standard Ministry classification. As Bathsheda and the local researchers struggle to decode the mysterious, razor-sharp carvings, Cassian touches a rune and is violently pulled into a vivid, historical vision of a powerful ancient witch named Yrsa, who wielded storm magic to seal a deep mountain cavern. Drawn deeper into the cavern, Cassian and Bathsheda discover a shifting spiral of runes leading to an abyss that snuffs out all light. Just as they realize the tomb is actively hostile, the ancient wards awaken, causing the cavern walls to fold inward like a closing mouth in a terrifying collapse. Working in tandem, they barely escape the crushing stone, with Bathsheda using her defensive magic while Cassian relies on pure survival instincts. The local Ministry officials attempt to confiscate their memories of the event, but the duo deftly evades the intrusion, returning to Britain with their secrets intact. This shared brush with death permanently alters their dynamic, melting away their academic professional boundaries as they surrender to a passionate, secret romance that deepens during a secluded getaway to the geothermal pools of Iceland. Back at the manor, Cassian uses the family's restricted library to study mind magic, realizing his transmigated soul and the system interface act as a natural firewall against the legilimency of Dumbledore and Snape. However, his peaceful research is cut short when his father, Regulus, confronts him about his relationship with Bathsheda, demanding he treat her as a mere political tool, forcing Cassian to make a choice between his family's dark legacy and his own path. He writes a scathing, defiant letter to his father, demanding the political space to operate on his own terms, setting up a high-stakes clash of wills within the noble House of Rosier.

The start of the new term brings the monumental arrival of Harry Potter, whom Cassian immediately recognizes as a neglected, underfed child rather than the golden hero the wizarding world worships. During the first-years' flying lesson, Neville Longbottom loses control of his broom and plummets from the sky, forcing Cassian to quietly deploy his newly unlocked ancient variant, Gravitas Ascendio, to freeze the boy mid-air before Bathsheda cushions his fall. When Draco Malfoy steals Neville's Remembrall and taunts Harry, Cassian ignores the strict school rules and encourages Harry to fly, witnessing the boy's natural prodigy firsthand. Harry catches the glass sphere in a spectacular dive, but the incident draws the fury of Snape, who demands Harry's expulsion for breaking rules. Cassian fiercely defends the boy, taking full responsibility for the flight and slapping Malfoy with a two-week cauldron-scrubbing detention, which prompts Lucius Malfoy to file a formal complaint with the Board of Governors. This political threat is swiftly neutralized when Regulus Rosier, furious at Lucius's attempt to undermine his son, pays a terrifying visit to Malfoy Manor, using ancient pureblood leverage and dark secrets to force Lucius into silence. Meanwhile, Cassian continues his secret tutoring sessions with Nymphadora Tonks, using highly illegal polyjuice therapy to help her master her volatile Metamorphmagus abilities under duress. His dedication to his students pays off as his pedagogical achievements unlock yet another ancient variant, the devastatingly powerful Apertis Oculus. On Halloween night, the peace of the castle is shattered when Quirrell warns of a troll in the dungeons, but Cassian's sharp instincts tell him the threat is a distraction, leading him straight to the first-floor bathroom where he helps Harry and Ron save Hermione from a brutal death. As the dust settles, Cassian realizes that the dark forces moving within Hogwarts are targeting something far more valuable than a simple school corridor, and he must prepare for an inevitable confrontation with the shadows of the past.

As the winter snows cover Hogwarts, Cassian uses his newly unlocked Apertis Oculus to bypass Dumbledore's confounding wards on the third floor, discovering the Mirror of Erised and witnessing Harry's heartbreaking grief over his lost parents. He chooses not to meddle, focusing instead on his classroom, where his bold lectures comparing magical traditions to Muggle scientific advancements trigger a massive ideological debate among the faculty, culminating in a fierce confrontation with Snape and McGonagall. Cassian stands his ground, defending the value of critical thinking and practical tools over blind, archaic ritual, forcing even Dumbledore to acknowledge his unique value to the school. The mystery of the third-floor corridor deepens when Harry and Hermione ask him about Nicolas Flamel, confirming Cassian's suspicions that the legendary Philosopher's Stone is hidden within the castle as bait for a resurrected Voldemort. During a spring outing near Hagrid's hut, Cassian and Bathsheda stumble upon a newly hatched Norwegian Ridgeback dragon, which suddenly goes wild and attacks them. In a stunning display of ancient magic, Bathsheda inadvertently speaks a primal draconic roar that subdues the beast, surviving a direct blast of dragon fire completely unscathed. Recognizing the signs of an ancient, forgotten Nordic blood bond, Cassian quickly uses the hatchling's blood to carve runes on Bathsheda's arm, binding the dragon directly into her life force and granting her permanent immunity to its flames. As the runic mark settles into her skin, they realize they have resurrected a form of magic lost to history, but the sudden flare of power sends a massive magical resonance through the castle. This ancient bond not only makes Bathsheda a formidable force but also ties them deeper to the unfolding mystery of the Stone, leaving Cassian to wonder if they can survive the coming storm as the dark forces lurking in the shadows prepare to strike their final, devastating blow.