They ask Menegroth for orders and then they open fire on every tree, bush, and blade of grass in the region.

After a minute they stop.

They do not go and get it. They do, since they are after all Elves, sigh and stare longingly.

The trees, bushes, and blades of grass react appropriately to this unnecessary violence, i.e. they don't do anything because they're plants.

The crown sits there. The jewels shine.

It's obviously a trap.

But it's such a pretty one.

The King announces that he's coming to the border to take a look.

The crown continues to sit there. The jewels continue to shine. They're so beautiful, aren't they?

Ten thousand troops arrive at the border. The King stops well short of it and takes a look. The Queen stops with him and does more than that. 

It is not an illusion, she announces after a moment. 

A woman fades into view, standing well back from the crown.

She, too, is surpassingly beautiful. She, too, is real. She stands straight and tall, utterly expressionless, and breathes quickly and shallowly like someone suffocating or in great pain.

The King goes utterly still.

The Queen stops operating her physical form entirely.

Not an illusion, she does muster after a minute. Some evil of the Enemy's, but real.

The woman is of course the perfect image of their daughter.

"Do you know the story of this crown?" asks the woman. Her voice, like her body, is identical to their daughter's but being used very differently.

We will not heed the lies of the Enemy, Melian says firmly.

"A sound strategy," says the stranger. "I did serve him once. Recently I came upon an opportunity to stop."

She smiles slightly. There is little that is pleasant about her smile.

"This I now swear," she says, "before Eru, before Manwë and Varda, before any other Power great or small who cares to listen. Melkor, whom you know as the Enemy, coerced an oath of service from me when I was a child barely old enough to speak. Never again will I allow that oath to compel me in word, thought, or deed. If my will is not free, then I shall go on without it."

Everyone stands there, stunned, horrified.

"I stole his crown and walked here from Angband in direct contravention of that previous oath," she says, a little distantly. "I'm not unprepared. But it is worse than I expected."

Swearing conflicting oaths puts you in constant psychological agony, so severe most people cannot do anything but beg for death.

Not an illusion, Melian says for the third time. The oath was genuine.

"He misses his crown and I don't anticipate you will like what happens if he gets it back," she adds. "I don't have the means to keep it out of his hands without help."

"I think we require more explanation of this situation than that," says the King.

"Where would you like me to start?"

"Your form. Explain it."

"The Enemy decided to make a copy of your daughter, because he thought I might turn out to have interesting powers and because he wished to be cruel to you. He never told me how he managed it, but I believe I represent a significant investment of resources and after what I just did I don't expect him to risk trying it again."

"When did this happen? How old are you?"

"Forty years ago, just after his return to this continent."

"What prompted you to come here now? Why do you oppose him? If you swore as you say, why are you even able to talk?"

"An unknown force descended on Angband and smashed the whole fortress to pieces. Melkor entrusted me with the repair of the crown. I decided I wasn't ever going to see a better opportunity to betray him. I oppose him because he is horrible. I am still able to talk because I have an immense capacity to endure suffering and despair."

"And what do you want?"

"My goal is the defeat of Melkor. Towards that end I would like to entrust the Silmarils to you until such time as they can safely be returned to their rightful owners, from whom Melkor stole them by murder and deceit; I think you have the best available chance of successfully defending them from him."

They're so pretty.

"What even are they?"