In the end, Duncan didn't figure out exactly what "Scions" were.
The goat-headed being was vague on this matter, and it seemed because it didn't know the details of these ancient beings that roamed the fringes of civilization either. As for Duncan, he could only summarize a concept from the limited clues—
Scions were the products of ancient times, and they harbored hatred for the modern world. They possessed weird and dangerous powers and stayed hidden and unassuming in the dark. Except for the Scions of the Sun, other "Scions" rarely appeared in the civilized world but threatened the safety of explorers in the peripheral areas.
And among all this information, there was one particularly intriguing point:
The Scions of the Sun seemed capable of disguising themselves as humans—only the transcendents of the Church could distinguish disguised Scions of the Sun from ordinary people.