Question about chapter 55

For context, this is when Vis and Ostius (My Goat? He actually moves the plot forward? He actually has balls? Is this unheard of? Holy fuck I love bro so much. If he's the villain I'm rooting for him) attack and then kill the military leadership. Vis adopts all the will that was imbued in the stones by the guys and then loses it when Diago kills them.

Questions. Why were they imbuing the stones to begin with? And why didn't Vis retain the Will? He kept the will from the chariot race, I remember. And even if they dies, it *his* Will now, no?