Looks like it's the author's take on Cagliostro's backstory from her 5* fate episode. Pretty nice. If you've never seen it, Cag's ability at alchemy was so good, Astrals tried to recruit her as a Primal Beast. She ended up stomping them, but an ambush led to her getting sealed away. We actually fight a rebuilt Adversa, and the old man laughs and brags that she was able to defeat the original; what makes anyone believe she would lose to a mere machine borrowing its power?
Cagliostro experiencing that feeling you get when you cheat in a game and become so OP that there's no challenge. It's super fun for a little while and then you just run out of things to do because you can slap things out of existence regardless of what they are.