True chameleons (2)

Considering the cultivation realm of the assassin at the time, even if he likely had a technique to handle the ashes without falling victim himself, the grade of that moth was likely to be one that was at the core formation realm. Any higher, and despite the measures the assassin had, the ashes would have likely turned on him.

As such based on the grade, while it was still just as lethal to a quasi-palace realm cultivator, it was perceptible to them.... under normal conditions. Luckily for that assassin, his target wasn't in a normal condition. The target was in deep seclusion, his entire concentration focused on combing through his cultivation so he could take that final step.