The direction of this nightmare was very clear.
Annan needed to find a way to break the ritual, not to continue it. That suggested that the nightmare didn't originate from Philip's faction... or rather, the Transcendents from the Corrupted faction.
Besides that, there were only two other factions.
Those were the official forces of the two countries, headed by Dmitry and Elizabeth.
In the original history, Annan inevitably didn't have the immunity ritual buff added by old Vasily. In other words, this ritual was disrupted while everyone was under ritual surveillance… yet still, it was sabotaged.
And as everyone knows, Ferdinand didn't die at that moment, nor was he wanted or exiled.
So, those Leystones, be it one or several, that were sabotaged in the original history must not have belonged to Ferdinand.
Who could notice a problem with the ritual without raising anyone's suspicions, and then sabotage it?