[ john's lack of obvious concern is appreciated. he's right: thomas doesn't much like being reminded of the vulnerability, the pain that causes it. even if he's the one bringing up the topic now, the memories. he doesn't mind telling john, though. at least not this much. ]

Yes.

[ after a moment, he elaborates: ] Magic in my world comes with certain risks. Stroke, mainly, and various medical terms that I've failed to memorise. If one does too much magic at a time, one dies quite suddenly. Staves help alleviate the issue. They store magical energy, if you will.