The matter of [going back in time] was nearly impossible to achieve in the mortal world, this principal material plane.
It would trigger a series of time paradoxes—if a person went back in time and killed their own maternal grandmother, they would no longer be born, meaning there would no longer be a traveler from the future to kill her grandmother.
Aside from this famous grandmother paradox, quite a few other similar time paradoxes existed, making the feat an insurmountable chasm.
"But as you know, being unfeasible 'in the mortal world' doesn't mean it's impossible in other planes."
"To give an example... in the distant reaches of the star realm, there is a world rift leading to the endless abyss. From that rift, one can peer into the interior of the abyss, where all time, space, and basic rules are invalid. No one knows what is actually inside there."
"According to magical theory, as long as one can acquire 'magical elements' related to the abyss, one can transcend the time law and accomplish the feat of [going back in time] in the mortal world."
The azure reaper stood on the rooftop, overlooking the heavily sealed football field on the ground and the securely protected portal in the center of the turf, and spoke to the person beside her.
"The only thing in the mortal world currently possible to have come from that distant abyss is, theoretically, that codex I shoved onto you. Did you use the ancient magic recorded in that book, little fox?"
"...More or less." The golden-furred fox maiden nodded, not daring to run scams and bluffs in front of the person beside her.
A living-fossil-grade senior, the azure reaper who could fight Red Rose to a standstill, trading blows evenly, Magical Girl ☆ Mercury, and the former