Did he seem to need to have the materials already on him at the time?

Yes, he wasn't conjuring liquids out of thin air or anything, but it didn't seem like they were already magical before he used his ?spells? on them.

And they were in, like, vials, he wasn't... licking things or bleeding on them.

Yeah. They'd be way more concerned if he was doing blood magic.

So it seems like it may be reasonably safe to let him talk and make a case for letting him into the Condemned if he'd like to.

Okay. Does he want to be present or should they handle it on their own?

He has nothing else pressing, he will come at least for the initial bit.

For lack of a better way to contain him while the army is on the march, the caster is chained up in a covered supply wagon. The person who gave the report to Blai has the unit's cleric heal the prisoner, then dismisses the cleric.

"Rise and shine," he says contemptuously.

 Blink blink. "—what?"

"Congratulations. You and your buddies had the bright idea of picking a fight with the Mendevian army. Feel like explaining what you were doing?"

 Not really but presumably 'no' is the wrong answer! "...........Adventuring."

"Uh huh. If you're hoping for us to let you live you're going to have to do better than that."

"We had reports of banditry out of the camp where I'm told you were engaged."

 "We never took anything from the army." Well, not much, anyway.

"Gotta say, you're not making a very compelling case."

"It remains a serious crime to steal from civilians," agrees Blai.

 "But giving someone their pay a month late, now, that's perfectly legal."

"Oh, do you want to confess to desertion too?"

 "No!" Some people are smart enough that they don't need to do a term of service in the army to guess how it'll go if they try.

"I have never paid a man late."

 He rolls his eyes.

Shrug. "Hey, I tried. If you want to hang I'm sure that can be arranged." He starts to move towards the wagon's exit.

 "—Hey, wait, no, I didn't say that — I can be useful—"

"Can you. I hear you have some kind of combat potionmaking ability."

 It's not that hard, anyone could pick it up if they actually tried, most people just don't bother. "I do, I can make fire potions, and I can make them eat through anyone's flesh if they're not wearing armor, and I can make myself stronger or more nimble or harder to kill—"

"And can you take orders, if given the chance to do so?"

 "Yes!" Probably. Unless he sees a good opportunity to get away, or he drinks one of the potions that makes it hard to think while he's already having a bad day.

"And how do we know we can trust you?"

 "...because you'll kill me if I don't? And because I need supplies to work, you can just only give me the ones you want me to use."

"Do supplies correspond very exactly to particular potion effects?"

 "They're not — they won't be too expensive, sir, I can earn my keep. But if you put in the wrong ingredients they'll turn out wrong, every potion is like that." He's not even sure most of his potions are magic but it's better if they think they are, and either way he'd get the wrong results if he made them wrong.

"So, if the same ingredients can turn into multiple possible potions, you must not only be theoretically useful but also trustworthy. Is that within your power?"

 "I'm not going to lie about what my potions do, that would be stupid."

"Not what he asked." The new rules are pretty limiting here but they don't stop him from looming threateningly. 

 "—Yes, I'll be trustworthy, I — I don't know how you want me to prove it—"

"The historical custom has been to swear an oath on induction into the Condemned. I do not, strictly speaking, require this, especially as deserting et cetera come with a death sentence whether you technically swore anything or no, but it would help."

 "I'll do whatever you say, I swear — is there more to it—"

"It is more elaborate than that, yes." Can the other guy recite it.