Now that the final boss video is out on Youtube, I am ready to reveal one of two things. The first thing is regarding the music for 2nd and 3rd phase of the final boss battle. The truth is, there is a lyric version of it, but once you hear it, well, you may not be able to "unhear" it for quite a while. Not that it's bad, but the singing version pretty much changed the perception of what the non-singing final boss music piece encompasses. Below is the singing version. The singing starts at 0:42.
In case you're wondering, yes, the voice is using Vocaloid. In fact, this song and most of other songs composed by Ciel use Vocaloid.
For the non-singing version and the actual final boss music, you can simply check out the Seraphic Blue final boss video under "Media".
As for the other thing? Well, that will remain a secret for now, because it involves me having to do something first.
P.S. By the way, in case you're interested, here are some other songs composed by Ciel: