I was asked for more info about Marcus and Dark Night. Marcus’ condition is not supernatural. Everything he faces has a precedent in nature. Of courses as a work of fiction, the attributes forced on him are somewhat enhanced. Even so, if a man existed living the life Gregor lives, the life he forces Marcus to live, in a time when science was not advanced enough to have the answers, Gregor would have been one of the main reasons for the vampire legend. Marcus is trying to understand himself, to understand what he’s been made to be. In a way, a more natural, human condition is far more frightening then a supernatural one, where your choices and behavior can be blamed on powers you have no control over. It is Marcus’ strength, given to him by his family, that gives him the ability to fight Gregor, to feel compassion and love. Marcus, not believing he is human, is more human than most.
The Calyptra is a moth that drinks blood from both animals and humans. This moth is found in many places including Europe. It is believed they descended from fruit eating moths and over time their DNA changed to make it the animal it is today. Their bites are rather ugly puncture marks reminiscent of the movie Vampire’s bites. Moths are attracted to light but often perish when they get too close to it. It is not a far stretch to say a human, with multiple bites could possibly get infected. Could this cause him to drink blood, stay away from daylight and perhaps even cause a long term DNA change. Who knows?
Add to that the uncommon allergy to sunlight that plagues a few people. In past times, again when medical science was young, these people were ostracized and forced to live away from many communities.
There are many animals that can regenerate damaged, even lost body parts. As Marcus says; “It is not so much that we cannot be killed as it is that we so quickly heal.” Hence what happens to Gregor. I won’t explain, you must read the book for that.
As far as strength and speed. There are many examples of extremely fast and strong animals. Forgive my slight exaggerations.
Then there is how Marcus lives and that is purely human, the best of human nature.