You could make like, a Mephistopheles alter or something along those lines that creates a singularity using a Holy Grail, perhaps tied to the Faust stuff. And so, that next singularity would be best accompanied by your Mephistopheles. And your story could go on from there ?
That is a cool idea for an unrelated event story.
The big problem is that my SI isn't a Master, he just works at Chaldea,
and unless you're also one of the main characters, no one GAF about including Chaldea staff members in anything other than very brief mentions,
and you have to get lucky to even have a canonical appearance.
That's my understanding of it.
