(Brother Sanders)
This one is just my opinion.
"and a viscous black liquid dribbles from the orb. Sanders drops his arms and hurries to pluck the gem up again, wincing when it pulls a muscle."
"pluck the gem up again" -- As written, I have issue with both 'pluck' and 'again' here. I get that he is once more taking the orb in to his hands after putting it down on the pedestal, but the last time he 'plucked' it was like 4 paragraphs ago. So while you can emphasise he is grabbing the core for a second time, I think it best to use something other than 'again.
As for the 'pluck', I think to be justified you need to make it clear why he is 'plucking' rather than just picking it up. I.e. you need to describe what he is plucking it from. I imagine he is plucking it from a pool of black liquid that is beginning to form, but the text doesn't actually mention that, it just says that there is liquid coming from the orb.
"wincing when it pulls a muscle" -- the natural thing for the 'it' here to be referring to is the orb. Obviously, the orb itself is not pulling a muscle, and even if it was (it is magical, after all) why would Sanders wince about it? Simplest change here is to change 'it' to 'he', but if you want to keep the intended meaning, 'wincing when doing so pulls a muscle'