"Maybe you can help me draft my letter in such a fashion as to avoid Leareth concluding that I and my god operate in the same mold and are impossible to talk with?"

....That sounds almost impossibly hard. But - important. And Vanyel - he's not sure, everything feels like quicksand right now - but he doesn't think he's given up, yet, on it being possible to communicate things to Leareth in a way that helps. 

"Of course. What would you want to say to him?" 

Leareth,

This letter was written with Vanyel's assistance, but I, Iomedae, am the primary author. I arrived in K'Treva Vale shortly before the explosion from what I believe is another world. 

I think that the existence of, and possibility of contact with, my world may change the justification for your war. I hope that this is so. 

I think that it will be much costlier for you to go to war with Valdemar while I am inclined to defend them against conquest. I am so inclined. You could persuade me either that this defense is entirely futile or that your war is justified, though only with great difficulty. I will almost certainly defend Valdemar if you attack at present. My presence on a battlefield is usually decisive. 

Vanyel has explained your grievances with the local gods and some, to me, seem legitimate. In the interests of honesty I must disclose that I am, like the Heralds, a selected follower of the god Aroden in my own world. The grievances Vanyel explained to me would not be applicable to Aroden. 

Aroden was a human once, and he represents that ascension did not change what he strived for, or his commitment to dealing rightly by others. 

Aroden ascended without killing other people, by a mechanism called the Starstone that remains in our world and still functions. Others have used it to ascend after Him. 

Aroden cannot operate in this world at this time. I will likely take steps to change that. You could ask me not to do that, if it seems to you that it might have greatly negative effects. 

I have no desire for war. It is among the most tragic of human endeavors. I have been at war for nearly all my life and am very good at it. 

The communications in this message are intended as part of a peace negotiation. Were we negotiating by the norms of the world I am from, that would have the following implications: I will, to the best of my abilities, not position myself to make inferences about your capabilities from the timing and process for the exchange of letters, and not use the contents against the aims of the peace process. I will not harm your agents in the course of their operations related to these negotiations. If you provide information or resources conditional on my agreement to terms I do not agree to, I will send them back. I will not lie. I will attest to the contents of this letter under truth magic, as relevant; I will swear not only that this communication is true to the best of my knowledge, but that the best of my knowledge is very good; that when I make claims in contexts like this one I am usually correct.

If you were to, for example, take this peace communication as a prompt to attempt to kidnap me, before knowledge of me would otherwise have reached you, this would be, in the terms familiar in my world, indicative that this kind of mutually cooperative and Lawful interaction will not work. I mostly expect that, regardless of your intent, this negotiation will not proceed along the terms familiar for Lawful negotiation in my own world, because we are alien to each other, and nearly certain to face much mutual incomprehension. But I am trying to make this negotiation possible for you and advantageous for you, and will do so past the first several points where it seems to me you are not doing the same. 

...He's impressed, is Vanyel's first thought. Leareth will be impressed, and that's an even higher bar. 

"He's going to have a lot of questions," he says eventually. "That's - probably fine, if you're thinking of this as the start of a conversation and not trying to hit everything up front, but - hmm. I think he's going to want more concrete claims about what you can do such that you're, er, usually decisive on a battlefield." Come to think of it, Vanyel also has questions about that. "Is it related to, er, how you survived the explosion?" 

"I have very fast reaction times, it's very hard to injure me, most injuries do not impair me, I can miraculously heal myself a dozen more times and under ordinary circumstances can do it sixteen times a day, most spells just don't affect me if I resist them, and I am unusually strong."

That does not on the face of it sound like enough to singlehandedly defeat an army, but - she definitely believes that it's true, and for the moment Vanyel is inclined to give her the benefit of the doubt. 

"...Huh. Is that all miracles from your god? - and are you sure still works, here, if you know you can't get more of the Healing?" 

"My health and reflexes have definitely remained, or the explosion would have been lethal. I suppose a possible inference from the fact that your truth spell worked is that my protection against magical control is gone, which would be - extremely concerning, actually -"

"....Yes, that would be really worrying. Leareth, er, uses a lot of compulsions." 

"Do you possess the means to check?"

"Compulsions do show up to mage-sight. Or, er, do you mean you want me to try casting a compulsion on you to test it?" He can technically do that but he looks so uncomfortable about it. "It might be fine? The first-stage Truth Spell worked but it - landed oddly - and I think maybe a coercive Truth Spell wouldn't work - I can just test that, if you want?" 

"A coercive truth spell obliges the target to speak? Yes, that should be an adequate test."

- and if he pushes more power into the Truth Spell, can he coax the vrondi to settle more deeply on Iomedae's mind? 

Nope. 

Vanyel sags slightly in relief. "Doesn't work. I can try the compulsion too, if you want, but - fairly sure it won't work either." 

"Good. I would not expect it to. And I believe my other Aroden-granted powers still function here -" She makes her magic sword glow EVEN MORE MAGIC. 

That's so satisfyingly dramatic! Stef is so jealous. 

Vanyel peers at the sword with mage-sight to try to figure out what that spell even is, but doesn't ask. 

"I think you should say in the letter that you believe you're immune to compulsions, and - I don't think it reveals anything we can't afford if you mention the part about fast reflexes and being very hard to injure?" He's curious about that, or at least would be curious if he had the energy; that's...not how Gifts work...mages are hard to injure but only because of shielding, and even Healers aren't harder to injure than un-Gifted, just more resilient to it. "He'll want proof of some kind, probably, or at least harder evidence than just your word for it, but - we can deal with that when we get there, I think." 

He frowns, skimming over the letter again. "...The part about information shared under terms you end up not agreeing to is confusing, it's not like you can un-know something he already told you? Though I think he'd be inclined to clarify terms up front before sharing anything sensitive, he's - very careful." 

"I can avoid acting on things that I was told in confidence, and routinely do so! But yes, I don't think we can or should try to jump right to - the way high-trust negotiations work in my world, it's just worth outlining what they'd look like in case he can recognize it. Some people can.

She will add to the letter, 

I am immune to compulsions. I am immune to fear. I have other forms of protection against magical attack which I do not expect at this time it's a good idea to specify. I do not know of a good basis for comparison across worlds, but in my own world the only powers that could survive in my presence on the battlefield for six seconds are halfway to being gods.

"Probably you should tell me a great deal more about - and perhaps demonstrate - how someone in this world would try to kill me, were they trying to do that."

She's immune to fear? How is that even a thing? ...It does feel thematically appropriate as a miraculous gift from the gods, but it's so weird. Also why "six seconds"? That's so specific! 

Vanyel nods his approval at her modification.

"....Er, if I expected you to be as hard a target as Leareth, and wanted to kill you. I'd - probably Final Strike. Which I can't demonstrate, sorry." 

"Is that - a bigger explosion than whatever happened in the village?"

"...I didn't see the, um, the explosion up close, I don't know how wide an area it hit." And wow he does not want to think about it at all. "Most mages - even Adepts - would, er, have a weaker Final Strike than mine. If it were me then...I wouldn't be surprised if the fireball and the crater it left was ten miles across." 

" - wow." Why is that a thing, that sounds incredibly terrible for the world and also she can't think where the magic power would even come from. ...probably you could do that much damage with a Wish, so it's just a Wish amount of power, and he is claiming he's much more powerful than most people. "I suppose the thing to say to Leareth is that I survived the village explosion and have every reason to expect I could survive a Final Strike for the same reason. ...and that if he wants to try it, he should notify me so I can make sure I'm ten miles away from any civilians." 

It's not actually funny at all and Vanyel is not going to snicker, the temptation to is nerves more than anything.

"Mmm. ...I do think he'd follow those terms, if you offer them and he - does want to try assassination first to prove for himself that you're not lying about your capabilities. I don't think he likes killing civilians, and - I mean, the whole reason he wants our country is to build up a denser population, killing a lot of people in the conquest would just delay that." 

She nods, like the kind of person who will notify you of their intent to assassinate you so that you can go somewhere less inhabited is a familiar type for her. "I'd expect his hesitation to be that I could prepare more for an assassination attempt if I'm told to expect it. Which I won't, obviously, but I wouldn't expect him to trust that I won't."