You could say that. Giving ordinary mages the power to call forth demigods and heroes is...more than most would probably have bargained for.

[It was sure as hell more than he'd expected, and judging by his best guess kayneth hadn't exactly planned for a nigh-uncontrollable monster. The less said about what became of Caster's Master, the better.]

It isn't, no. Sometimes, should a Master be killed, a Servant can manage to persist on their reserve magical energy long enough to find another before disappearing. But they can't do so indefinitely, so a situation like Caster's should be impossible. Given the amount of the ambient magic in this city, however, I'm not entirely shocked that it's possible.