Unfathomable to mere mortals is the lore of fiends.

Taken at face value, the line asserts a sharp boundary between ordinary human understanding (“mere mortals”) and the secret knowledge attributed to demonic or corrupt forces (“fiends”). In Hawthorne’s moral universe, evil is often linked with hidden motives, masked intentions, and a kind of perverse insight that tempts or manipulates the innocent. The sentence can be read as a warning against presuming we can fully comprehend—or safely traffic in—the logic of wickedness: to try to master it is to risk being mastered by it. It also reflects Hawthorne’s recurring theme that some regions of the soul and of moral experience remain obscure, resistant to rational explanation.