And we might. ...breaking out definitely destroys the possibility of them interacting with us...

Yeah. Bit of a last resort, that one.

One last catch. Your father mentioned that married couples don't or can't keep secrets from each other. I don't think Elwe has a way to use the knowledge against us?

Other than becoming a practitioner and being a stupid one. That'd fall on Melian, which - might hurt Doriath? Also, dunno if the married couples-can't-keep-secrets problem applies to a Maia married to an Elf...

So there's a couple layers of protection even if there were something he wanted to try.

"Your Majesty, we have the explanation you asked for, and something to offer you."

No idea if she's listening.

The door decides to go back to existing a little while later. 

Looks like an invitation.

Who's on the other side?

The King and Queen, together. They don't have guards, but it's rather obvious Melian would hardly need them.

Your Majesties.

Your explanation.

It is long and implausible, but you will understand why we were evasive. I am wiling to periodically swear I haven't lied to you if necessary.

She tells Melian the side effects if she decides to pass it on to Elwe and Elwe the fact that informing people is dangerous. And, as politely phrased as possible, that she only promised one explanation.
Alongside the (distorted) set of karma rules, she offers to make Melian a practitioner if she wants it. She describes the demesne ritual as giving near-total influence over a location and making it unassailable.

In a respect different from what we already have here?

In a surprisingly similar sense, actually. 

The traditional description, though it's probably an exaggeration, is "one step below a god." And you're already you.
You could expect everything you do affecting Doriath to be easier, and more successful, and to accumulate good luck from managing it the way you currently do. I suspect that if Doriath were your demesne and the Enemy himself came to your borders, you could turn him away. Or let him enter, fight, and win.

If nothing else, you could easily make it so he can't locate you by having someone fly overhead.

Please explain how this is done.

The using a demesne, or the finding Doriath?

Using a demesne.

It's supposed to be very intuitive, almost as simple moving your own body except with less feedback and more gradual. Whatever you most often want your demesne to be, either consciously or not, it shifts. It has an easier time with minor contradictions than what you currently have seems to; it would make perfect sense for the sky to be visible from below but not the ground from above. Whatever you currently spend attention to maintain you could have happen automatically.

In and around the area, you can expect the spirits to side with you in any conflict where you have a side, and to obey your requests or orders. If you use the practice for anything it will be safer and more powerful than usual, but even if you don't the universe will cooperate with your actions. Usually I'd say it can allow reshaping the setting at will, but that you already have. 

Yep, there's still a door behind them.

Some powerful practitoners, which would definitely include you, can reenter their demesne from a distance should they choose to leave it. That one will have more to "how" than just telling it to happen, but it varies by individual.

I would like to learn this art.

One more warning that doesn't apply to most people.

When a practitioner isolates their demesne, it can fall between the cracks and fade from the world with them inside it. You will probably have very good karma, and will not be actively dragged down, but this is not a good risk to take. Especially with a kingdom's worth of people here. To avoid it you should make sure that you and your demesne stay connected to people and places outside it.

I can think of three ways to do this. One is to periodically leave and visit other places personally. I suspect you'd be unwilling to do that. A second is to claim only part of Doriath as your demesne, making sure that much of your population is in the part not claimed. Or you can maintain ties to other kingdoms by sending and receiving representatives, engaging in trade and diplomacy, and setting up channels of distance communication.

Over what sort of time scale is that a risk? Is falling out of the world harmful to the people involved, would I notice if the process began?

Extremely harmful. The Abyss is a mostly literal place, with a claim on all things lost and forgotten. It would grind you down and destroy you, and this is not something you want to risk. The site that used to be the demesne would be a savage place, holy and enchanted, but only saturated with power and no direction.

Before any of that, you'd notice that you seem to be getting weaker. Weaker as a practitioner; I don't see how it could affect the power you have already. It would happen if the spirits stop flowing between the inside of your demesne and the outside, which you'll also be able to see directly. If that starts to decrease, it would be tempting to improve your demesne to fix it but that would not address the problem. It's about maintaining ties to the outside.

As for the time scale, it can happen in Years. Years as measured in the world outside; you'll be able to alter the flow of time if you like at the cost of counterintuitive, unpredictable jumps for people who exit.

Thank you. We can give some people an assignment of regular travel to outside for as long as the continent remains safe, at least. After that we can still have regular communication with anyone but the Enemy. If the Enemy conquers the continent, though, I am not sure how we'd meaningfully maintain connections of the kind that you say is required...

It is entirely possible for connections with an enemy to count. If he's besieging you and his lieutenants are plotting things to try against you, the world won't be able to forget Doriath is here. You would probably have to actively take the fight to them occasionally, so it doesn't turn into routine and unthinking stalemate.

If you'd like to become a practitioner, we have most of what's needed for the awakening ritual.

I would learn more of it, or have you swear that's all I need to know.

I've said all you need to know to do it safely. Learning more would increase your capabilities with it, and would mostly be limited by what I know. I'd be happy to tell you what I can either before or after you awaken.

"I swear I haven't lied to you," she adds, since I guess before now you couldn't trust that practitioners can't lie.

What is involved in acquiring this magic?