Our duel is much like an endless waltz. The three beats of theory, denial and re-evaluation continue on forever. But it's time to bring this to a close.
Imma powering up to my final form!!!
The night before the Third Twilight, by hook or by crook Lancer managed to get into Ilya's guestroom and killed her there. Then he proceeded to go to both Caster and Bazett's guestrooms, convincing them in turn to let him in and murdering them shortly afterwards.
He then set up the crime scenes of each of their rooms, working backwards by leaving his own key in Bazett's hands then locking her door before moving onto to leave Bazett's key in Caster's hands and locking the door before finally returning to Ilya's room and left Caster's key in her hands before locking the door and taking Ilya's key with him. He then hides himself, either in his room or in the corridor - somewhere where he could observe the Survivors heading to the Banquet Hall in the morning, and after he saw all of them go he headed to Ilya's room and set up the painted sigil on the door and then moved onto the others while being careful to avoid detection by the Survivors whether by precise timing or hidden compartments in the corridor or whatever. Once Lancer was done with Bazett's door, he returned to his room and used the inner lock function to seal it as he no longer had his own key. After the Survivors found Bazett's body they advanced upon Lancer's room with Key in hand.
Lancer lay in wait, ready to fake being a corpse with his partner in crime declaring him deceased. However, disaster struck for Lancer when the door opened and only his partner was standing there with a Stake in hand - immediately stabbing him and then closing the door and locking it with the Key. They then turned to the rest of Survivors who were turning the corner towards the guestroom and apologised for running on ahead but they definitely heard Lancer screaming. They then whipped out the Key and unlocked the door to find Lancer's body with Ilya's Key thus creating the Locked Room Chain...
Phew, transformation of Blue Truth complete. How do you like my 100% form?