Rift
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Who killed Kostyan: IMHO - it was Neira.
Now, regarding Kostyan's murder - a magical artifact-clock, awakening its eighth owner (???) and a strange death - upside down and significantly aged...
As far as I know, bats usually sleep upside down, vampires tend to turn into bats, premature aging - sudden, immediately by several hundred years, as if someone drained his life, you could say, dried him out... Cash has a link to a vampire clan in his profile, besides, in his "hoochie Coochie Man" - in the song by Chizh & Co (the title of the first night is reminiscent of "Night Musician") there are the words:
"Oh, I'm Hoochie Coochie, oh, I'm Hoochie Coochie Man
Roll my body, mama: I'm completely dehydrated." - is this just a coincidence?))