Weiss was already aware of this special explosive item of the royal family as it was something that was used during the siege of the Triston family. The reason why their sturdy outer wall was breached so easily was because of this exact item.
Weiss had actually gotten her hand on one of these through the Seven Heroes Conclave. This was actually not an exclusive item for the royal family of the Brightshine kingdom and many powerful people throughout the whole world had access to these.
But these things cost a lot of money. So even for a old royal family, having too many of them was not possible. If Weiss had to take a guess, about a few thousand should be the maximum.
So giving this hidden knight named Risty three of these explosives, that was the proof that the Royal Family was taking this dark orginisation quite seriously.
