I'd complain we didn't have much of a chance to see through the trick and not to murder our friend, but I'm pretty sure that was intended to be railroaded.

(Sylvia was last seen standing ahead of us, by the throne. We wouldn't expect her to teleport to the rear corner of the room. Even if killing her with the axe was an illusion, we did then follow up by flooding the room with lightning. But... no body on the floor in the last few panels.

Really not sure then if we killed her, or more importantly, when the illusion started).

Maybe the illusion started when the quest started. Or when they entered the throne room. Or the illusion just started.

I did railroad that bit with the illusion.
I won't however do it next time.
And if it makes everyone better, Cynthia also suffered from an illusion.