I can see one downside of voluntary marking for visitors as potential social consequences upon return. If the mark is easily visible in any way, there are going to be plenty of people hostile to the realms, their gods, and anyone connected to them.

Going around with a mark in some parts of the world is going to be like wandering Alabama in a 'Hail Satan' t-shirt. I can even imagine many strictly Islamic countries would regard it as apostasy and declare it a capital crime.

There are also going to be a lot of rumors going around. That people who get the mark will be bound to the eternal service of their God after death. That accepting a mark lets the God control your mind and soul. Doesn't matter if it's true or not, many tourists aren't going to take the chance.

And good old arrogance, of course. Plenty of Important People who are full enough of themselves to sure no God would dare mess with them.

The more seasoned realm-dwellers may make fun of these visitors, call them 'tourists' and enjoy some quiet satisfaction whenever one of them ends up force-marked after ignoring all the warnings. At least such people might lead to occasional work when someone with actual wealth and influence gets caught and the family/company want to hire a mercin- ah, 'local contractor' to rescue the idiot. Or find out what the church might desire as ransom.

This is indeed a component, but I feel it becomes a bit weak in some ways.

Not all marks are visible most likely, and beyond that, since the narrative focus is on Nathan and the realms, "people in the USA don't like you" is an offscreen weakness.

For Nathan, sure. But it's not the player characters that this issue applies to - it's the NPCs. Or for some character perhaps their origin story.

Concerns over social exclusion and rumors of soul-stealing provide an explanation for plenty of un-marked NPCs to go traipsing around in the Realms, like ambulatory story hooks. Adventurous tourists, academics looking to get a research paper out of the place, business-people seeking to exploit the commercial possibilities, zealous missionaries keen to convert some actual heathens. Some smart, some idiots, some well-intentioned if misguided and some who see the realms as a new source of exploitable labor. All of them ripe for getting dragged into the story.

Or it may just be that the gods grow a little annoyed if someone volunteers for the mark and promptly runs away back to the mundane world - it strikes me as something highly disrespectful towards the deity, to seek them out and then turn one's back upon them. The gods may have no direct power over those outside of the realms, but they are not above sending one of their more loyal followers across the border to teach a little respect.

Part of it might also depend on how interested the gods are in events outside of the realms. With no direct power they would rely entirely on agents to represent their interests, and marking might play in to that. Take a mark and leave the realm too soon without intention to return, and you have a debt unpaid. A favor that may one day be called in. And it is a bad idea to say no should this favor be called in, because the gods have their debt collectors.

It's not likely to happen because most individuals are simply beneath any divine notice. But it's something anyone who takes a mark would have to worry about. Sometimes the finger of fate will reach into the world, and sometimes it's the middle finger.