Archwizard Clark had once said something very famous:
"...If an elderly and virtuous wizard says something is possible, then he is probably right. But if he says something is impossible, then he may be very wrong."
"...The only way to discover the limits of what is possible is to cross those limits, running from the impossible into the possible."
Rat Immortal greatly admired those words.
Because more than one "elderly and virtuous" old wizard had told him that certain things were impossible—
There was no saving his wife, who had already fallen into a Mi-go. The best possible ending was for her to grow old and die in the Black Prison. He could never develop a new, more sophisticated Magic Ritual to reverse her fall.
His daughter, lost in the Mirror World, was already dead. So much time had passed; he should let go. Even if he traveled through mirrors hundreds of times every day, he would never find a soul destined to vanish.
And there was the ratfolk he had created with his own hands. This was no longer the era of centuries ago. Creatures suited to being elevated into the extraordinary—foxes, bats, wolves, and the like—had long since been claimed. Rats were merely ordinary creatures. He could not, like the progenitor of the Blood Clan, forge his Ancestral Seat by creating an extraordinary race, then break through to become a legend.
Since so many "elderly and virtuous" old wizards said it was impossible, that only proved his choice was very likely to succeed. Rat Immortal thought this, becoming ever more resolute. He even abandoned his former True Name, appearing before others only as "Rat Immortal."
And so.
He hid beneath First University, painstakingly nurturing his ratfolk while spending his spare time traveling through every mirror in the school,