Mithril
The first thing we must conclude is that the song of the Roots of Hithaeglir is apocryphal. For this topic, it is unnecessary to thoroughly analyze the tale itself,* so we shall leave it with Patrick McKay’s words.
But also, we know Elrond is a lore master, and he is aware of this tale. He says in that fifth episode that it’s apocryphal. I would trust his read on a piece of lore.
Patrick McKay
* I explained a bit of why The Roots of Hithaeglir is a fabrication, on pages 6, 7, and 8 of Sauron: The Perfect Inverse of Beren.
Reflected Light
What exactly was wrong with mithril? It was always peculiar and valuable, but for different reasons that Celebrimbor believed, who said it contained the light of the Valar.
“Nothing diminishes its light” (Celebrimbor, 1×05).
The first episode provided the perfect template for us to determine the source of its light.
“But sometimes, the lights shine just as brightly reflected in the water as they do in the sky” (Galadriel, 1×01).
The Roots of Hithaeglir implied that mithril contained the light of a Silmaril, and therefore the light of the Valar (a good source). However, the first time we ever saw mithril, it was suggested that its light was “reflected” (signs of a nefarious source). But the Dwarves knew its true qualities.
Durin IV: “Lighter than silk, harder than iron, as weaponry it would best our proudest blades. A specie that might be dearer than gold.”
Elrond: “It is strange how it catches the light. It almost seems lit from within” (1×04).
We see from the conversation between Galadriel and Finrod that reflected light is intended to disguise the darkness of the water; the darkness of the water intends to master its victims and drown them. Reflected light is convincing, in that it shines as brightly as real light, but it is best avoided.
[Gil-galad]’s got this connection with the life force of Middle-earth, almost as if he can feel the tentacles of evil slithering beneath the crust of Middle-earth. And he knows it’s there. And so, it begs the question, What am I going to do about it?
At this point in time, Sauron was the source of deceitful and perilous things, though he would not have created mithril, no more than Morgoth created gold. The one who created it would have been Aulë, who created all elements in Arda. But mithril would not have always harnessed the power that allowed it to heal a blighted leaf. That is where Sauron entered the picture.
Perilous Whisperings
In every episode of the first season, there was a particular sound of eerie whispers—a total of 17 instances verifiably associated with Sauron (19 if you include its musical equivalent in the tracks Perilous Whisperings/An Intriguing Suggestion and True Creation Requires Sacrifice). The video below shows 14 of those instances.
In each case, the whispers seem to signify some form of sorcery, specifically Sauron’s. The hilt was crafted by his own hand, so each time it was activated or examined, we heard the signs of sorcery. The same happened when his signature mark would burst into flames, when Adar explained Sauron’s attempt to craft a power of the unseen world, and when Sauron, in person, infiltrated Galadriel’s mind—sorcery.
Additional evidence lies in moments with the Stranger. (Bonus connection between the sorcery of these two Maiar at the end.)
His whispering was similar. Sometimes he whispered aloud, sometimes additional voices whispered with him, and often we heard both. The difference between his and Sauron’s whispers is that we can often discern particular words and phrases with the Stranger. Sauron’s voices are darker and unintelligible—they sound perilous.
Having developed an association, we may assume that the remaining five moments represent Sauron’s magic.
The first clip is subtle, technically only a part of the full sound effect—instead of whispers, the air sounds warped; the black and white clip shows the same sound accompanied by the whispers as Sauron warped Galadriel’s mind. The last clip shows the first time we hear the Forging Theme (the beginning of the track Perilous Whisperings) where there might have been a whispering sound effect.
(One particular instance is being saved for page 7.)
Why would mithril be associated with Sauron in this way?
Sauron, knowing that the Elves feared extinction, poisoned the Great Tree to exacerbate their fear so they would accept the means of mastery that he would offer, a.k.a. the power within mithril. [inhale] In so many words, he identified what his opponent most feared, then gave them a means of mastering it so that he could master them.
By his own hand, he stopped Galadriel’s attack and infiltrated her mind. By his own hand, he carved a mark into Finrod’s flesh and the anvil in Forodwaith. By his own hand, he fashioned a power for Men in the form of a sword hilt. By his own hand, he crafted the power of the unseen world found in mithril. Sauron could not be the giver of a gift if the gift never belonged to him in the first place.