Minaiyu does not say more, not immediately. He's standing there, brow furrowed, prodding at the vocabulary.

There is...kind of a lot of technical jargon in here, actually. There are words in here for things he only knows about from medical training. He doesn't get results for everything he looks for, and some of the terms are just taking the obvious roots and connecting them in the obvious manner, but...

"...Marcus, do you know what an ossicle is? What about an aneurysm?"

"... No. Why would I know what an ossicle or an aneurysm were?" 

"--because they're in your head. I reached out for your words, and they were there."

"They're not in my head. Except in as much as you have brought them there." 

Also he's not entirely happy with the idea that the demon is stealing his words but what's he going to do about it. 

"Then where did they come from? I didn't speak this language before I got here!"

He turns to the witch, looking lost. "Does this make any sense to you, honoured one?"

"Well, you could be a demon with the ability to speak in tongues and a bizarre agenda for deceiving us all? But assuming you're generally telling the truth? No clue. Not even sure why you're so certain you can read his mind, really." 

"Not his mind, just his language.

It-- normally, when you lodge in someone's brain-- it doesn't always happen, sometimes you have to learn the language the hard way, nobody's sure why-- but sometimes their language sort of bleeds over to you, and you can share their ability to speak it.

But it's always their language as they understand it. If they don't know a word, you won't either."

But also bees can't grow that big and humans can't do magic, so really, what does he know!

"Well, clearly you misunderstand something about your own nature. It happens to the best of us." 

"The important point, though, is that we're all convinced that Marcus going mad and obtaining inexplicable knowledge of medical lore in the same moment seems rather implausible, so we can assume that Marcus is sane and you're a real entity?" 

He looks pointedly at the onlookers and receives affirmative gestures from them. 

"Then, we have to rule out that this is a deceptive possessing spirit of some sort. Fortunately, this is a solved problem! I can't cast the spell, but I have here" The witch rummages around in his bag to retrieve a small container. "An unguent whose effect is to temporarily suppress possession or other mental influence from malign beings. I would be happy to contribute this towards the benefit of your family, here, but this stuff isn't cheap, and I'll have to demand compensation for it."

Marcus's father sighs. "How much?" 

"Three sheep." 

"That's highway robbery! Do you want my family to starve?"

"I have to buy this stuff from Old Blacktooth upriver! It's not cheap!" 

"I'll give you one sheep. It's that or nothing." 

"You'd pay two sheep if I was selling a potion of Cure Light Wounds for your son on his deathbed." 

"And you wouldn't charge that, because you'd have spell slots open. I'll give you a sheep and a jar of honey." 

The witch sighs dramatically. "Very well. A sheep and a jar of honey, and I'll use this unguent to drive any demons from your son." 

This is giving him snake-oil vibes, but again, what does he know.

(He still can't use the snake oil as an excuse to pretend to go away, not without buy-in from Marcus that he doesn't expect to get.)

"...drive, or temporarily suppress?

And, uh, does it work on non-malign beings?"

would they be able to tell the difference between it driving him off and it rendering him a silent passenger for decades

"Temporarily suppress! But there's a second half of the plan you don't need to know about. All of the things it works on would be doing the possession intentionally." 

He's not actually going to stop the process of dragging himself out of a sitting position in order to give a hypothetical demon a chance to talk its way out of this. Have some weird-smelling goo on your face (and relatedly, +2 to AC vs. attacks from demons.) 

...yeah, that's fair. Minaiyu really can't blame him.

He wrinkles their nose at the smell.

"I'm still here," he says.

(He is still kind of wondering if this is even a real medicine. But if Marcus and his household end up with a justified true belief that Minaiyu isn't malicious, well, he supposes he can make that work. All he really needs at the moment is for them to let him have the chance to prove with his actions that he's a mutualist and not a parasite.)

This witch is wishing he'd bought those mix-ins that let you add glows and sparkles and fizzes to your potion effects if they don't have them natively. The things we do to save money. 

Urgh, that's a terrible smell. 

"So. Now what? You're not a demon or an enchantment, apparently? How do I send you on to your rightful destination?"

"...well, that's not...normally what happens is that the dead soul stays with the host for the rest of their life, and when they die, the two of them--separately--wake up in new hosts somewhere else. There are people back home who have been through this several times.

I'm...not sure what it would mean, for a destination to be rightful. Although, well, there isn't anyone back home who's done it dozens of times, so maybe eventually they all end up in worlds with heavens and stay there."

He sighs. "I'm sorry. I know this is all very disruptive. I'll make it easier wherever I can."

It's very disruptive for me too, he doesn't say.

Marcus is silent for a bit. He's considered the problem of making friends with things other than animals and family members but has not yet had a chance to put theory into practice. 

"Well. We've been terrible hosts for you. We should all go in and talk over breakfast. It's not right to keep an innocent guest standing out in the cold."

That last bit is addressed at his father, whose reluctance to let this situation go on without Doing Something is only overcome by the horror of possibly being seen as a bad host. 

He nods, smiling a little awkwardly at the self-deprecating apology. Some warmth would be nice right now, as would some food (though the anxiety and general turmoil has been largely drowning out the false-tiredness).

He turns to the witch and gives him an acknowledging nod. "Thank you, honoured one."

(It doesn't seem like this person still holds the power of life and death over him, at least not actively, but it's still best to err on the side of being polite.)

[People may have noticed, by this point, that Minaiyu's voice is a bit different from Marcus's. It's subtle enough that if you weren't expecting two people you wouldn't catch it, and even then it's still entirely plausible that Minaiyu could pretend to be Marcus if he actually tried, but it's otherwise pretty doable to tell who's speaking.]

He holds the power of life and death over everyone here! He's the only person in two hours walk who can cast Cure Light Wounds! But you can't go pointing that out to people too much. They get resentful for some reason. 

"All is well that ends well! If you two need to get away from this lot, my door is always open. ... metaphorically speaking, even I don't want this weather in my house." 

He's going to hike off before he is overcome by the desire to sit back down again, because that would be the bad sort of undignified.