I'll note that, despite what people have been saying, there probably will be externally visible signs of something going on with the shadow knife matrix. Internal bleeding leaves signs you can see from the outside (swelling and massive discolouration, if I remember correctly), and a shadow knife going off inside someone is going to cause a lot of internal bleeding if it's not an instant kill (and maybe even if it is). Note that this isn't a bad thing necessarily, as it means a closer investigation of the body is more likely, but it does mean 'looks like natural causes at first glance' likely doesn't apply.
So then the ideal construction would be to use magic, but kill leaving no traces simulating natural causes, but take care to make it appear as if it was due to hidden unknown vampiric matrix spell, arranging it to be discovered?
Probably not much place for a new plan this far into voting, especially when the race is so tight, but for the sake of discussion and maybe approval voting it could be something like this:
[X] Plan: Nighttime Completion
-[X] Sneak into the Tzar's room when he is sleeping alone. Use Mockery of Death on him. Implant him with a Matrix loaded with Sleep, setting the Matrix off immediately. Asphyxiate the Tzar to death in his mockery sleep, leaving no mundance evidence. Remove Ulgu traces to the extent it is possible, except for traces of the Matrix, making sure they are discoverable and making them appear as if it was Dhar-tainted vampiric version.
*Matrix with Sleep just cause it needs a spell inside.
Satisfies both Boris' conditions:
Do it as quietly as possible, leaving no evidence. If the way you do it could only have been done with magic, that is good.
As if a Lahmian had done it while doing their best not to leave evidence that a Lahmian did it.
The plan leaves no mundane evidence and could only be done with magic.
And it leaves Lahmian-pointing evidence in the form of Dhar-looking Matrix traces. As if the Tsar was set up to appear to have died from natural causes by killing him during sleep by an unknown vampiric silent killing spell, presumably implanted during the day or even several days ago, set to go off remotely and when the killer is no longer around, so that no one would suspect a thing. Suitably Lahmian, except for assumed slight slip-up of vampiric Matrix traces, exactly as requested.
The conceivable failure mode would be if no magical examination would take place to discover the Matrix traces, but with
a) this being an unexpected death of a healthy and very active head of state, so definitely needing a check,
b) relatively recent vampire-related disturbances in the realm, and
c) the heir having suitably good relations with both the priesthood, hag and ice witches,
a priest, a hag witch or an ice witch or any combination of them would definitely check the Tsar's body and find them.
The plan aims to 100% Boris' conditions, relying on Mathilde's sufficient Grey Wizard Lord skills and capabilities to execute it and manage its risks and complications.
---
Still approval voting for the current leading matrix plan.
[X] Plan: Nighttime Heart Attack