The invisible person detected by the talisman master was Amon. His invisibility was due to an invisible talisman.
A standard invisible talisman had a low-tier runic grade, similar to the one used by Gui Xa, the royal assassin who had attempted to assassinate him in the past. A higher grade existed, which allowed the talisman to mask the user's presence against Spirit Stage cultivators, like the one used by Sima Fang. However, the one Amon used was of a lower grade than theirs. His invisible talisman was of the high-tier engraved grade, which could not hide his presence from a cultivator's sense. That's how the Mystic Stage talisman master could detect him.
