Colorless and tasteless, blending into the wind, even those in the refining dirty realm would find it difficult to detect it for a moment. By the time you realize it, the poison has already taken root deep within!
For most poisons, their effectiveness diminishes for those in the Marrow Tempering realm and can even be transported out of the body by manipulating one's qi and blood in the Visceral Tempering realm.
However, all of this takes time, and when martial artists engage in combat, they seek opportunities within those fleeting moments. If your combat capabilities are lowered in the present, that's the greatest victory.
On one hand, Qu Yuanen tried to persuade, but in reality, he had already ordered someone to poison the scene. Whether Chen Fei refused or agreed later, all the initiative was in Qu Yuanen's hands.