"Not a whole lot is actually known about the Infinity Stones. At least, not by me—Strange probably knew a lot more. They've supposedly been around since the creation of the universe, though I'm not sure if that refers to the actual rocks or just to the singularities that they contain. The Space Stone can...do anything you like that's related to space, if you have the strength and control to wield it—if I picked it up and told it to do something, it would probably just teleport me randomly into intergalactic space. I think we'll have better luck trying to integrate it with your existing magic system, like we did with the Time Stone in New York, although—under more controlled conditions and hopefully with fewer unpleasant side effects. I'll ask Fëanor about it, but the Fëanor we have here just learned to smelt iron fifty years ago, so if we really need his input we'll probably have to just risk going back to the present to talk to that one."

He goes to get Fëanor and the Tesseract.

I couldn't really tell you anything about it, Fëanor tells Tony and Leareth-via-Nayoki. It's obviously very powerful, but the power doesn't really seem to have any structure—it's like someone took ten thousand times the power of the Trees and compressed it into a point.

(Nayoki relays this.) 

This is not exactly among the more helpful explanations that Leareth has ever heard. 

"Noted. I suspect that Velgarth mage-sight would be able to see more detail and structure. Nayoki, could you...?"

"Yes, of course - I had already been planning to examine it for you."

Leareth nods. 

...Then makes eye contact with the local, younger Curufinwë Fëanáro. 

:- I would like to speak with you, if that is all right: he sends, privately. (It takes a lot of willpower not to wince visibly. Mindspeech hurts.) :Privately with Nayoki, that is, she can translate for me: 
 

Yes, I've been wanting to do the same, he replies. Then, seeing Leareth's wince at the mental contact, he says aloud in broken Valdemaran, "If osanwë hurts you, know-I little of language-yours. Would-like-I more learn." He has no accent whatsoever, and his command of the language is shockingly good for his very limited exposure to it, he's just, clearly, substituting his native language's grammatical constructs where he doesn't know the Valdemaran ones. "Or osanwë-my may-translate-she," he adds, pointing to Nayoki, "if understand-not-me-you."

Leareth would like to speak to me alone, he says to Tony.

Tony leaves the room.

...That's pretty impressive. 

"Before this, I spoke with your - older self, in the other timeline," Leareth says. "And your son, Nelyafinwë Maitimo, who - told me of some things that might be of interest to you." 

Nayoki waits to see if a translation is needed. 

(And tries to read Fëanor's mind, if she can get away with it.) 

The younger Fëanor hasn't yet had need to acquire the habit of keeping a tight lid on his thoughts; Nayoki can get a lot, though not everything.

Nelyo told you interesting things? he sends to Nayoki, deciding that this conversation is going to be beyond his command of Valdemaran, at least on the speaking side. Nelyo doesn't care about anything he finds interesting, he likes hanging out at Court, and networking, and politics, gods forbid. He spends more time in the city than at home, which is fitting, actually, since Fëanor ran away from the damned Palace when he was about the age Nelyo is now—

What did he say?

Leareth is so unsure how to say any of this. 

...Also he's suddenly realizing that he's feeling very disoriented - Nayoki told him that they came to Valinor, and he knows some of Valinor's history from Maitimo, but not nearly as much as he would like. 

"Where is Melkor, right now?" he asks. 

Nayoki relays this. 

Imprisoned in Mandos for three Ages of the world. Of which one and a half have gone by.

:You should keep him that way: Nayoki sends, darkly, before translating for Leareth. 

"...Right. There - was another war, later, after the Valar released Melkor. It...went badly, in many ways. You - your future self - had made the Silmarils by then - they were captured by Melkor - you swore a magically binding oath to protect them at any cost -" 

Leareth's head is pounding again and he's not sure he remembers all the events in the right order. 

He takes a single, long, slow breath.

None of that surprises me. The Valar will release him when his sentence is done: they are not the sort of creatures that could break their word, whatever the cost, nor can they even comprehend the danger that Melkor poses. And if the Valar prove themselves so foolish, then of course I would plot to leave Valinor; and if I need to leave Valinor, of course I would preserve its Light; and if that Light exist in portable form, then of course Melkor would try to steal it; and if Melkor steal the Silmarils, then of course I would recover them at any cost whatsoever. The Valar have the strength to defeat him, but cannot see him for what he is; I can see him plainly, but I have not the strength. Perhaps, before the time comes, I will acquire it.

He pauses as a realization dawns on him.

The Infinity Stones. They were used to kill half the population of the entire world, in your time. Could they kill a Vala?

:- They did. Several of the Valar died, when it happened: 

The Valar, of course, would have told him that wasn't possible, wasn't even coherent. Now, if Leareth is telling the truth, they've just been proven for liars—and if they're lying about their own natures, they're probably lying about everything. Fëanor's perfectly happy to believe that Eru created the universe, on account of the fact that it had to come from somewhere, and perfectly happy to give Him due honor for that, but he's never seen a single shred of actual proof that Eru created or authorized the Valar. If not—and it seems at the moment to be not—there's no reason to let them rule over the Quendi. His people could become, one day, just as powerful.

With the Infinity Stones, even more so.

Before you take the Infinity Stones back to your own time to fix the emergency there, he asks Leareth, would you mind if I used them to kill Melkor? I will probably leave the others alone, for now, although I may wish to secure an assurance that they'll let us leave when we're ready.

"It does seem likely that would avert many tragedies. I...will certainly consider it. ...Also, please do not swear any magically binding oaths. Even if it seems to be a good idea at the time. I promise, I will bring all the resources I can to bear, in order to solve problems that may arise, but... My impression is that the oaths your species is capable of, are much more trouble than they are worth." 

It would be much appreciated. And I suspect you are—missing context, on the oath. To give up future optionality over something like that is a very high cost with no obvious benefit. My future self would not have done it unless there were—

He pauses suddenly.

You said you were working with me, in the future. Why did I not come with you? The Silmaril was definitely his work, but it could have been stolen—

"Elves cannot, it seems. I believe that neither can the Valar or Maiar. You are - bound to your own timelines. The first test subject for the process was an Elf, and it killed him." Leareth bows his head, his voice apologetic. 

Then the Valar were not lying about everything, he says sadly. They have told us that we are bound to the fates of Arda, while the Secondborn will have free will but be, consequently, bound to leave the world after a while. However, it was supposedly the same fact that made the Valar unable to be killed. Perhaps the Infinity Stones are...exceptions to the usual rules, somehow. Speaking of which, you said you would like to examine this one? He gestures to the Tesseract, which Tony left behind when he left Leareth and Fëanor alone.

The Tesseract, the cube-shaped container for the Space Stone, looks at first like a cube, but by turning it just right, one (who knows as much math as Leareth) can see that it's actually the projection of a higher-dimensional cube-analogue onto three-dimensional space. Directly visualizing the higher-dimensional space in which it really exists is impossible for human minds, or even most elvish ones, but by observing the twisting of the cube as he turns it Leareth will be able to see it obliquely. From just the right angle, he can see the tendrils that extend from the singularity of power inside the Tesseract to every point in space. This is...some kind of Master-Node—responsible, apparently, for the very concept of whereness itself, and its power is as vast as the universe.

Leareth is a lot better at mathematical visualizations than most people. He still can't quite wrap his head around it, but he has enough to work from, and the geometry of it doesn't make his head hurt even if actually using mage-sight does. He gets in a peek anyway, until the tightening band of pain around his forehead warns him that he's pushing too hard too soon. 

"Nayoki, can you...?" 

Nayoki is finding that trying to look at the Tesseract's geometry is making her head hurt, but she can try to get a closer look at those tendrils. 

Is there any obvious way to interact with them, using her magic, and follow one to a specific place?