When the Antiquarian's region of terror ended, Lot was there. He was just an Inquisitor then and while he had a strong reputation and and the only person to volunteer to interact with the serial murderer, no one expected him to survive the encounter.
But if the High Inquisitor were asked he would tell you that he wish he hadn't. Because then the exchange that occurred between the two wouldn't haunt him to this day.
It seems that the Antiquarian "took a fancy" the brilliant young detective and promised one day to return to the city and "add him to his collection". A promise that was bound by the mask that the fiend placed on Lot's face to mark him as his own. A mask that Lot cannot remove to this day.