The Second Magic, also known as the "Kaleidoscope," was fundamentally the proof and application of parallel universes.
Through the phenomenon of multidimensional refraction, it enabled free travel and the exchange of matter between parallel universes. Its core mechanism involved observing differences between world lines for precise positioning and interference. It also encompassed the concept of time travel, allowing records to be changed and events rewritten.
Zelretch was a wielder of the Second Magic. He could move freely between multiple parallel universes while preserving his own identity. So long as a world contained ores and gemstones, he could travel there.
One of the worlds—or rather, works—where he appeared most often was the world line of Tsukihime. Around AD 300, war had broken out between the Mage's Association and Crimson Moon. Zelretch had forcibly united the mages, who preferred acting alone and lacked any military organization, earning him the title of Wizard Marshal.
Although Zelretch had repelled Crimson Moon, the Third of the Twenty-seven Dead Apostle Ancestors had ambushed him during the battle. He had defeated his attacker, but had his blood sucked and become a Dead Apostle, leaving him unable to oppose Crimson Moon and making him an enemy of humanity. However, he had already intended to retire, and had never been a friend of humanity to begin with, so he took the opportunity to vanish from the stage of magecraft.
The other was the timeline of Fate/kaleid liner Prisma Illya. He had created Magical Ruby and Magical Sapphire based on Kohaku and Hisui, then sent Rin Tohsaka and Luviagelita to Fuyuki to collect Class Cards, resulting in Illya and Miyu becoming magical girls—
"Teacher, let me explain—" Zelretch felt helpless, too. He saw himself as an old alumnus who had graduated from the school called Earth. As long as the