Sinsystems
Patron Librarian
The issue is that the request was for the method to be a way that can only be done with magic and the method doesn't do that. Yes a shadow dagger could be the cause or the assassin just moved aside the clothing before stabbing (or what if the Tzar sleeps bare chested?), yes magic could have caused the Tzar not to struggle or it could have been some poison or anesthetic.Mathilde will stab them because shadow knife goes through non-magical armor, so the untouched clothing can serve as a really subtle clue (one we can tell Boris to use as a justification, if necessary) that something fucky went on.
Why would a Lahmian do it with magic rather than a knife? Because not carrying a knife allows you to feign innocence. During infiltration, if spotted, you could go "I am but an unarmed woman" and have a pretty decent chance of gettting away with it. As for the shadow dagger not being a common mastery? Fair, but she could just throw a normal one into his heart. Also, other people also have unknown magic. Boris doesn't require a crime scene he can perfectly reconstruct using known magics. He just needs something he can use to point at his targerts (on whom it seems he already has dirt in other ways).
I get the idea, but I feel like the current plan leaves too many ways for it to be done without magic.