#anyway bonus mini-theory: callum wasn't 'overwhelmed by darkness' or whatever because the coin spell is unusual in its reagent

[kradogsrats]

So this spell has been a big ??? since s3... particularly for me, personally, because it's an outlier that fucks up a lot of possible explanations for how dark magic corruption and/or the Staff of Ziard work. It wasn't super unusual for it to not have a visible reagent or incantation in its first appearance with Viren—they don't bother with with runes and incantations for a lot of incidental or mid-battle spells—but having seen another couple more examples... well, here's a thought:

This would have been an extremely useful spell for Claudia at multiple points during Arc 2, if not in Arc 1, right? At the least, if she could do what she did at the Moon Nexus during her confrontation with Callum, Rayla, and Ezran in the Sea of the Castout, she'd still have her leg. Now, that would have been a massive escalation of power for her in that scene, but the writers on this show don't generally hamper characters for no reason at all—I think it's entirely reasonable to conclude that, for whatever reason, she couldn't cast this spell at that time. If we knew that it required a very rare reagent that she didn't have at the time but could have gained access to in the intervening period (like, idk, unicorn bone dust), that would explain it... except we've seen pointedly and repeatedly that this spell has no incantation or reagent.

Something significant does happen between s5e9 and s7e6, however:

Let's pull out Tales of Xadia for a second and look at some of the dark magic mechanics:

If you have any degree of Corrupted trauma, you are considered to be somewhat permanently marked by dark magic. This has the effect of granting you some abilities that neophyte dark mages don’t have.

The examples of this in Tales of Xadia are kept low-key, like lighting candles (which we see Viren do in the series and Claudia do in Puzzle House... though with a very different explanation, which believe me, I am definitely spiffing up my best my tinfoil hat for right this instant) and sensing other spells or sources of magic... but "oh, and you can immobilize anyone, up to and including an archdragon, with a thought" wouldn't be a very smart power to open up for dark mage player characters, so tbh that doesn't surprise me much. But I think that's what's going on with the incidents of this spell that we see in the series: it's only possible if you've fucked yourself up pretty dang badly with dark magic, already.

This could also explain why the spell itself doesn't increase personal corruption in the mage, but appears instead to have some more mundane debilitating physical consequences, instead—it's not drawing on an external source of essence, but operating on something entirely internal... whether the mage's own living essence, or something else. (Insert more tinfoil here. It's very shiny.)

But yeah, this isn't a spell... it's an ability that becomes innate to the mage, once they've done some level of permanent damage to themself with corruption.