Annatar crouched low to the ground, pointing out the insect flitting about. Far behind them, Ost-in-Edhil's lanterns shone against the night's darkness, but his young pupils had left their lamps behind.
His pupils! He had never thought to take students, and Melkor had used the promise of a forge of his own, free of novices to entice him away from Aulë's workbench, all those long years ago in Valinor. Still, Celebrimbor's youngest son showed some skill in metallurgy, a gift Annatar would not see squandered if he could help it. He supposed the way Curungil's friends pestered him for stories of adventures was a small price to pay for the potential of youth. He supposed.
So he had indulged them, for Curungil's sake. He had few adventure-tales fit to be shared with such tender ears, and in any case, he had always found nature's majesty more awe-inspiring than any deed done by elves or men. But he had showed the lads how the daisy's petal unfolded in ordered perfection, and how alike that proportion was to a pinecone's spirals. Curungil had seen with his own eyes how that same ratio was found in certain varieties of quartz but not in others, and Annatar had privately been impressed by how well the lad grasped the underlying concept.
They had moved on quickly to refraction indices and the varying crystalline structures and how to judge a specimen's quality by sight alone. They had spoken, too, of just why some gems tended to display diasterism and others did not. They spoke of the inner nature of things and why some raw materials melted under intense heat while others were ruined by it. Curungil's eyes took on a fierce blaze during those conversations, reminding Annatar of the stories he had heard of Curungil's forefather. He was Fëanor's heir indeed, and in more ways than one! Annatar was reminded, too, how he had once relished discovery when the world was young. When he was young with it.
Behind them, Curungil's friends let out a whoop as they chased each other around the campfire. Their cries were akin to mûmakil storming through the underbrush, trampled the moment that had so captivated Curungil. The boy looked longingly toward his friends, and Annatar inclined his head, silently giving him permission to leave if he wanted to. Curungil nodded his thanks and ran off before Annatar could withdraw the offer.
Annatar heard himself sigh in spite of himself. Beside him, Celebrimbor chuckled. "He is only a lad, you know," the elf said quietly. "For all his prodigy at shopcraft, he is still a child in other regards." Then, fixing Annatar in his sharp gaze, he added, "I know a heart still beats within your chest, Annatar, though you try to hide it. You are fond of my son. There is no shame in that."
Annatar waved his hand, as though he could shoo Celebrimbor's words away as easily as he could the glow-worms. "I never claimed otherwise. But a heart does not excuse indulgence where it is not warranted. I would not spend my time on Curungil if he did not show promise, for all that he is your son."
"Why do you encourage his questions?" Celebrimbor asked. "I have known you to bristle at much older apprentices, and your time at the forge is precious to you. Why have you taken to Curungil?"
Annatar sprang forward and captured one of the glow-worms between his fingers. Cupping his hands together, he watched as the glow-worm flew around the confines it found itself. He lifted his thumb just enough for Celebrimbor to see inside, though the insect was still held captive.
Celebrimbor took the proffered glimpse and studied it for a moment, but to no avail; when at last he looked up at Annatar, confusion was still plain on his face. That was discouraging but not wholly surprising. "The glow-worm burns," he said, "but he is not consumed by the fire the way a faggot of wood might be. There is no source to its light – seemingly. What would it be like, to unravel its secrets?"
Celebrimbor looked at him suspiciously. He opened his mouth as if to voice a question but then closed it again without uttering a sound. At last he asked, "It is bioluminescence, is it not? You explained that to me once, and it seemed no great mystery."
"True enough," Annatar said, "but where does that leave us? Say there are chemicals within the glow-worm that give light without end. How is that possible? Affixing a fancy name to the phenomenon hardly qualifies as an explanation."
"You seek after a flame imperishable?" The color had drained from Celebrimbor's face, and Annatar was surprised to see genuine concern on his friend's face.
Opening his hands up, Annatar watched the glow-worm fly off into the night. "You make it sound such a radical notion," he said. "What do you think science is, at its root, but the pursuit of the spark that birthed the cosmos? We would be heretics, each and every one, were there room for heresy in truth."
Annatar looked down at his hands and saw how the moonlight reflected in the adamant set in his ring's silver band. Would that he had found the secret of light that needed no fuel for its promulgation! But he still had much work to do on that question, if he could ever hope to answer it. So far he had been unable to capture what was not already inherent in the gems he worked with, and true ringcraft required a more intimate act of creation than that.
Looking back up at Celebrimbor, he offered the elf a brief smile. That was a rare enough gesture for him, and he hoped it would serve to appease Celebrimbor, for in truth he suspected he had spoken too openly before. "It was not his quest for the flame that turned Melkor's heart," he said, "but the solitude of the Void. It is not good to spend too much time alone. And, to answer your other question, that is why I tolerate Curungil's youth. He is precisely the sort that Aulë would have drawn to himself in before-times, and so he reminds me of something I thought I had long lost within myself. Quite aside from an artisan's duty to nurture talent, I find myself strangely refreshed when I spend time in his company."
Celebrimbor nodded, seemingly accepting both aspects of Annatar's answer. A part of Annatar wondered, did the greatest scientist the Noldor had produced in these later days grasp what he had left unsaid? If he did, Celebrimbor did not push the topic. "Just do not take Curungil too far down the secret paths with you," the elf said. "Too much truth too soon can be a dangerous thing."
Annatar was not so sure about that, and it goaded him to let others set terms on him. Still, Celebrimbor was the lad's father, and there was a natural authority in that relationship that Annatar knew better than to interfere with. Besides, for all he felt refreshed in Curungil's presence, he was not a teacher at heart. Annatar knew he needed assistants with more learning (to say nothing of experience) than Curungil could claim.
Someone like Celebrimbor, perhaps.
"As you wish, adar," he said. "But I will need a companion of some sort on 'the secret path', as you put it. Will you walk them with me?"
Celebrimbor did not answer him. Still, Annatar could see that the possibility had planted itself in Celebrimbor's mind. For the moment, that was enough.
In addition to providing valuable beta-reading assistance, pandemonium_213 also served as the inspiration for this short treatment of Annatar (Sauron). I read pandemonium_213's treatment of Sauron in Eregion before I read Tolkien's, and I must admit I quite prefer the former to the latter! (Not that they contradict each other or are even really comparable in that way; they simply have different focuses.)
I hope this story is comprehensible without further knowledge (of the finer points of canon, or of pandemonium_213's stories), but if you haven't read those stories you are missing out on a real treat. They are listed under "Eregion" here. This story was particularly inspired by "Cat's Paws" and "Risk Assessment."
On how much the elves of Eregion would have known about Annatar's true background:
The only canon I have found touching on this point is in Unfinished Tales, where we are told that "In Eregion Sauron posed as an emissary of the Valar, sent by them to Middle-earth ('thus anticipating the Istari') or ordered by them to remain there to give aid to the Elves." That last phrase in particular is interesting. It suggests that Annatar was sent to the Elves, so he was not seen as part of them. Of course, this might be read as meaning "the Elves of Middle-earth" (as opposed to the Dwarves or Men), but it seems just as likely that the Elves of Middle-earth knew he was not an elf.
At any rate, I have a hard time imagining the elves of Eregion could be fooled in this way. Sindar and Noldor both lived there, so if he claimed to be of one of those kindreds or of the Teleri, you have to think someone would be related to whatever family line he claimed when asked about it. As for the Avari, I suspect Annatar's scientific skills were too similar to those practiced by the Noldor for that to be convincing. It seems most plausible that Sauron was either vague about his origins or that he claimed to be one of the "good" Maiar. That is just my interpretation of some rather scant (quasi-)canonical evidence, though. Other authors may read the situation differently.