Is it in song form?

Nah, then you'd need someone to keep singing it. She gestures at the ground and it resolves itself into a brick; she draws some symbol on the brick with her fingernail, which briefly glows red-hot. Boom. A Power has to contest it to get past it, and contesting every brick in a wall takes forever, more than long enough for the little guys to run.

Loki takes an illusion copy. So, how tall does a wall have to be to work at all?

I mean, they usually also build them tall enough to keep orcs away. If you don't have many bricks, doesn't take that long to contest, but other than that there's no limit.

Does anything need to happen besides this symbol being in the wall? Does it have to be engraved? Does anything besides engraving have to happen? Is there anything to be gained from multiple symbols per brick?

Do I look like an Elf? Or an architect?

None of those told me about this in the first place!

You're not a Maia, so they wouldn't work on you! Or maybe the Elves who remembered what the last war was like didn't come to fight this one, sensibly so, and somehow in Valinor the knowledge of how to keep Maiar out with your walls didn't get passed down. Or perhaps Melkor spread misinformation about it, actually, that's a thing he'd do.

So you don't know the answers to my questions?

I've never built such a wall, never played around with it too much, all I know is what the wild Elves had back beside Cuivienen. It was enough to warn them so they could flee, it was the first magic they learned. Perhaps someone kindhearted told them. They were fucked enough as it was and it could have been even worse.

What do you want to test walls for me?

He hasn't sent me any in a while, but if he sends me unarmed orcs and they go home when told I will tell them to go home.

Then I'd be happy to pop by and test your walls. When?

Well, one of the tests I want to make is whether I can just wallpaper the place in the symbol and have that work or if you can just walk through it by contesting the illusion and not the symbol. We could try that here.

Hit me.

So Loki makes a wall of the symbol, patterned prettily in midair.

She walks through it. Damn. Nah, don't even have to contest the illusion. I think the engraving-in-brick thing must be doing some of the magical work.

Okay... Loki breathes ice into her hands, winds up with something vaguely bricky, etches the symbol into that to see if it spontaneously melts or anything.

It doesn't spontaneously melt. I could contest it by melting the ice instead of by contesting the symbol, though, betcha.

Be my guest. Loki places the ice brick.

She focuses, and starts melting a thin line to connect parts of the symbol. I'd be incovenienced by a thousand of those, but someone with heat as an ability wouldn't especially.

All right, so it wouldn't help if a Balrog comes to call. Loki grabs a rock, turns Lævateinn into something etchy, etches.

That's the version I'm familiar with. She goes bat, tries to flutter over it; the symbol glows and the bat flaps in midair and she concentrates for several seconds before it stops glowing and the bat flutters forwards and immediately back into a woman.

Is the symbol used up permanently now or would you have to do that again to cross again?