She'd be using substance of shadow which achieves invisibility while in shadow, unless you're saying there's no shadows available in a room at night? If she's discovered before the Matrix can be performed, she can simply use Shadow knives at Vladimir to kill him like the normal plan. There's nothing complicated about it, Mathilde can hide from anyone in the castle, so multiple attempts at the Matrix shouldn't be an issue.
We're in Kislev, it's cold. The Tzar most likely has a fire burning in his room. And what if someone comes in holding a candle, or lets in torchlight from the corridor? Light fucks up Substance, and the thought of being under the effect of it for an entire hour honestly scares me.
And even if the room was pitch black, Substance still wouldn't work, because it only works in shadow, not darkness.
K / Substance of Shadow: A single person or item that falls within a shadow becomes invisible, silent, and insubstantial, though they can selective choose to physically affect the world. Lasts until the item or character is illuminated.
- Must specifically be a cast shadow, not merely being in darkness. Think shadow puppets.
- Must be a single discrete object, not a selected portion of a larger object.
- Anything longer in any dimension than a meter, or heavier than 10kg/22lbs, will be much more difficult to cast this upon; anything more than twice that is approaching the limits of possibility.
- When cast on an inanimate object, it will only become insubstantial to anything touching it after the spell is cast. Anything it was acting on before the spell, it will continue acting on after the spell.
- If something becomes substantial while 'inside' another object, results will be thoroughly unpredictable, usually unpleasant, and sometimes dangerous.