The critical truth of this murder is that it was in fact planned and executed between the cases of Alex Bracknell and King Arthur. The Blackened, drilling into the chemical storage case in the Chemistry Lab using an electric drill taken from the Research Roof, discovered various chemicals- but most notably, they discovered a bottle of sodium cyanide, which they took for themselves. They then travelled to the Obelisk’s internal chamber, and noticed an empty pedestal there. At this point, a plan formed in their mind.
Taking a blank paper note they had found in the Senate and a pen from the Chemistry Lab, they wrote a false label for the cyanide, referring to it as a “Brilliant Potion” which would greatly increase the intelligence of anybody foolish enough to drink it. Leaving it on the pedestal in the room, they hoped to make it seem as if it were what was hidden inside of the mysterious pillar. And so, the murder weapon lay there, acting as bait that wold hopefully create a mysterious death that nobody would be able to trace back to the Blackened.
The killer’s plan stood in wait while the class unravelled the mystery of King Arthur’s suicide, and in the period after the trial, the Ultimate Witch, Matilda Martell, wandered into the Obelisk Chamber. There, the victim noticed a mysterious liquid with a label that promised greater intelligence. Leaving their glitter on the pedestal, they took the murder weapon, and planned to drink it in front of others later, for whatever reason. The reasoning behind why the victim chose to dramatically swallow the liquid in front of other people is irrelevant- all that matters is that as exploration of The Tomb’s Halls of Non-Euclidean Geometry were underway, they drank the cyanide.
Five minutes later, the poison asphyxiated them, leaving the dead body of Matilda Martell in the same room as Arthur Townsend… and the Blackened. With glee, the Blackened realised that their plan was successful, and they had succesfully killed the Ultimate Witch without having to lift a finger. Nothing except careful analysis of handwriting could expose them, which thankfully did not occur until it was already too late.
That’s how you did it, wasn’t it…