Not knowing much about Tsukihime, this is really good to know. I always thought of Zelretch as a Shiki-level big deal in Nasuverse, but didn't realise that there was a branching point leading to these actually being two separate universes altogether as a result of his actions - fascinating stuff. His parallel universe second magic does fit very nicely with the multiple ones in Nasuverse, Deus ex machina levels of storytelling convenience heh. Which is why I'm surprised that I've so far only seen his involvement in the FGO universe in a very desirable craft essence or two heh.So yeah this guy basically split the Fate universe away from the Tsukihime universe when he fought the moon (don't ask). It's pretty straight forward, if Zelretch loses to the moon (don't ask) Tsukihime happens, if Zelretch beats the moon (don't ask) then Fate happens. But right now we have two clear different universes, meanwhile Zelretch himself is the sorcerer of the second magic that directly links to the root.
Prisma Illya is one of the Fate series that I've never been very interested in picking up, but is it worth watching, in your opinions, if the story and lore in it are interesting?