Threat

Arriving at his mansion, Eijiro immediately went into his private room to make a potion that could neutralize the poison in Katsuki's body.

 Katsuki's body had indeed improved and recovered a bit, but it wouldn't last long if a next level antidote was made for Katsuki. Katsuki will experience another decline in health if he doesn't get the next antidote soon.

 Eijiro hadn't said this to Shouki and Katsuki yet, afraid that the two of them would be even more anxious and make Katsuki's body react even more. The effect of the poison, Katsuki's magical core, and the panic Katsuki was experiencing were intertwined. Therefore, it would be better if Eijiro was silent for a while until the antidote was made.