I want to remind everyone that Nasu didn't even properly split Fate and Tsukiverse until the middle of last decade, and it wasn't official til we got this gem from the afterward of case file volume 6.
The director of the Atlas Institute, Zepia Eltnam Oberon (or Atlasia), was originally one of the twenty-seven special vampires known as the Dead Apostle Ancestors in Tsukihime.
Though, as has been already touched on in earlier works like the drama CD that came with the volume of Chaldea ACE recently and the Bamboo Broom diaries, the universe in Fate is fundamentally different from the Tsukihime universe.
Speaking of that, it really was a big surprise… After all, Kinoko had already chosen all the authors for the spin-off works way before the first book of Case Files was finished. Now, time for a reenactment of what happened (all the lines of the authors have been completed).
Kinoko: Actually, the twenty-seven Ancestors didn’t become the twenty-seven Ancestors in the Fate universe.
Authors: ……………!!!!???? (All freeze, speechless)
Kinoko: Only the Tsukihime universe allows for them to become the twenty-seven Ancestors. On that note, doesn’t Tsukihime not have the sort of extremely powerful familiars like Servants?
Sanda: What about CM—!?
Kinoko: That thing gets destroyed because of the destruction of the Human Order. Oh, also, Narita’s Fake is different, because it’s a special middle ground between the two.
Narita: O-Oh, uh, t-thanks…?
Kinoko: But Sanda’ll have to pay attention for the Case Files. It’s kind of a long story, but this part in Tsukihime R….
Sanda: Waitwaitwait! Kinoko, wait! Please first organize some notes about this for everyone!
Kinoko: Come on… so annoying. Speaking of which, this pizza’s pretty good.
Higashide: Oh ho ho, either way, my work is already done, so take your time.
Sakurai (carefully): …It also seems to not affect me at the moment, huh.
There probably hasn’t been a day that shocked me that much since then. Ah, no, I do remember that…
If you read it along with the fourth book of Fake, you might be able to tell that there’s something different between the two universes. Comparing world settings with Narita and others and playing spot-the-difference is as enjoyable as the experience the night before a cultural festival.