"A mirror person"...
Hearing that term from a god—or rather, half a god—who had fallen over a century ago and whose true body could only fight the corruption within this mausoleum made Angel's scalp prickle. Somehow, it felt inconceivable.
But then she remembered that the emperor before her had once ruled all of Intis, including its capital, Trier. He must have thoroughly investigated the city beneath which the ruins of the Fourth Epoch's Four Emperors War lay buried. He might well have suffered some form of corruption in the process, creating a special "mirror person" of his own.
According to Danio Brown, the mirror person she had discovered replacing an infiltrator among the Nighthawks, many places beneath Trier leaked corruption. Prolonged exposure to them would produce mirror people with independent minds in the Mirror World. They had long since formed a closely coordinated organization, constantly replacing their real-world "originals" and taking over various positions, seemingly in preparation for some major operation.
Angel had reported this to the Machinery Hivemind and the Purifiers. The official Beyonders had conducted a roundup, clearing out large numbers of undercover mirror people. Since then, the organization had rarely shown itself.
Could mirror people have been secretly lurking in Trier over a century ago, luring Roselle, then consul of Intis, underground and exposing him to corruption without his knowledge? Or had Roselle explored out of curiosity himself? Given his personality, the latter was more likely... After privately grumbling about Roselle, Angel finally asked for confirmation:
"You went to Underground Trier and came into contact with the corruption there?"
Roselle nodded and said:
"It seems you know quite a bit about that place too... After I overthrew the Sauron Family and became consul of the Intis Republic, I personally explored Trier's Underground Area to expand the city