PrePub - Ascendance of a Bookworm

At first I wondered if the law only allowed a duchy attacked by black weapons to fight back with black weapons of their own, but not their allies... but then I remembered that Ferdinand and Eckhart are still officially Ehrenfest nobles, so any such contingency clause would apply to them...

Maybe the law against the use of black weapons against anything other than feybeasts and -plants doesn't have any exceptions, because the Zent at the time was naive enough to assume that no large group would dare defy the law so brazenly, that the defenders wouldn't have time to ask for permission from the royals. If that's the case, I can see why the allied forces would be reluctant to use black weapons out in the open, at least while they still had other options. Rozemyne and Matthias would attack from within the building, so their actions would be hidden from view far more than that of the knights attacking from the air, making them far easier to hide.

As for overloading the tool: With how Grausam has already drawn absorbed mana from his arm to shoot back at the attackers, overloading it would likely be a considerable effort and risk. So it's more of a last resort, if Rozemyne's sneak attack fails, rather than the first go-to thing to try, I would assume. They also have to make sure they don't completely exhaust themselves just to deal with the arm alone, or they'll be vulnerable to any other magic tools Grausam might be carrying. To assume he came only with his prosthetic arm and two packs of Lanzenave poison would be beyond naive, especially for someone so cautious it borders on paranoia, like Ferdinand.