User blog comment:Octofan/Original Sin Story -Act 2- Booklet/@comment-3467648-20130907233021

So, apparently Kyrill was part of Project MA, somekind of assistant for Seth. After see the consequence the Project caused between his sister and his fiance he begins to doubt of his own sister and Seth's ambition. Irina doesn't seems to be the same, as if HER finally took over her.

Although his musicbox is broken he stills hears it, but instead of a melody it was a voice (Elluka?). Fearing the worst he took the determination for stop the project and bring back her beloved one making a ritual using a forbbiden artifact.

Really I don't know if Kyrill had HER and Elluka's death and his siter's betrayal make him snap or if Elluka had HER and the second personality seduced him to unleash the Fire Disaster with the 'Sin' artifact... whataver it was at the end the real Elluka could do nothing as she was resurrected and gained the immortality as consequence of the ritual held by Kyrill