After Marcius's prediction, Maria did not speak up for a while. Maybe she was shocked because of Marcius knowing the exact reason why she gave the bone of the Spirit Dragon to him in the first place.
And it was as he said. The bone of said dragon had the capacity to rouse an extreme reaction from those who had physiques associated with souls in some way or another. And Marcius's king physique, the Soul Devouring King physique, which was the highest caliber among the soul physiques of Rubrum, would have the strongest reaction to the said bone.