Warning: spoilers ahead for The Rings of Power episode 5Middle-earth's Second Age broadly fits J.R.R. Tolkien's legendarium.
The Rings of Power episode 4 then saw Durin IV tell Elrond that mithril represented a brighter future, but the extraction process involved great danger. King Durin III was also terrified the Elves would learn of Moria's new mineral and covet mithril for themselves, which means Elrond gets forced to sign Middle-earth's equivalent of an NDA.
Through The Lord of the Rings, audiences knew Elves would discover mithril eventually, but this lone detail aside, The Rings of Power's mithril backstory seemed over. About that... The Rings of Power episode 5 ("Partings") reveals a huge secret behind mithril that completely upends Middle-earth mythology and makes the ore way more important than anyone would've guessed.
How Mithril Was REALLY Created In The Rings Of Power
The Lord of the Rings: The Rings of Power writes a totally new origin for mithril. According to Son of Fëanor fulfilling his familial duty.
According to the Elves' legend, this elf poured his light (more on that later) into the tree to protect it. The Balrog countered by barraging the tree with its darkness. Along came a lightning bolt to strike the tree and combine the Silmaril's light with the nearby forces of good and evil, sending a wave through the mountain. This process created a special ore within Moria that shone with the glow of a Silmaril, possessed the strength and fortitude of a Balrog, but had the weightlessness and grace of the elf. The Dwarves of Khazad-dûm would later discover this mineral, and mithril was born.
How The Rings Of Power's Silmaril Myth Rewrites Canon
The Rings of Power's new mithril mythology is a fairly egregious rewrite of several aspects from J.R.R. Tolkien's work, starting with how a Silmaril got itself stuck in a tree. The Silmarils' movements are well-documented after their arrival in Middle-earth: two were kept by Morgoth then promptly lost forever upon his defeat - one under the sea, the other deep beneath the earth. The third Silmaril was taken from Morgoth and ed from one owner to another (never further east than modern-day Lindon) before ending up on Eärendil's head, and The Rings of Power has already confirmed the tale of Elrond's dad is TV-canon. When and how a Silmaril wound up in a tree atop the Misty Mountains is, therefore, a mystery.
The elf-warrior in Gil-galad's myth could be a Son of Fëanor, and the Balrogs were Maia corrupted by Morgoth, so the battle itself checks out fine. Alas, the elf pumping his magic light into the tree has a less obvious connection to Tolkien lore. The Calaquendi were elves blessed by seeing the Two Trees of Valinor, and they became more mystical and powerful than kin who never witnessed those glowing trunks. The Rings of Power interprets the Calaquendi more literally. Middle-earth's live-action Elves are spoken of like batteries filled with light. They can dispense this gift unto others, but eventually need recharging in Valinor.
But by far The Rings of Power's biggest change is the reframing of mithril as a creation borne from three forces of Valinor - light, darkness, and a Silmaril. In Tolkien lore, mithril was naturally occurring - just really useful stuff the Dwarves pulled out of rocks.
What Gil-galad Means By The Light Of The Eldar Fading
The Rings of Power's mithril retcon is such a grandiose explanation for what's fundamentally just a chunk of ore - good ore, but ore nonetheless. Mithril plays a small-ish role in Middle-earth's history - the Doors of Durin, the Baggins' shirt, how the Balrog was awoken... and that's pretty much it. And yet in the name of creating a new origin for this substance, The Rings of Power reworks both Silmaril history and the Elves' biology, as well as spinning a whole new First Age legend. It's like going to the trouble of purchasing an entire house just because you quite like the shed in the yard, but The Rings of Power's illuminating mithril lore actually serves a deeper purpose...
Gil-galad and Celebrimbor drop a massive secret on Elrond: the Elves are gradually fading away. As The Rings of Power explains, living in Middle-earth away from Valinor's light is bad for the Elves' health, and when the glow inside them fades, the Noldor themselves will apparently diminish. There are two possible solutions to this pointy-eared predicament - return to Valinor and bathe in its light all over again (but abandon Middle-earth to Sauron), or find a source of that light somewhere in Middle-earth. Mithril is that alternative source, and if Gil-galad can convince the Dwarves to part with enough of the stuff, they can stay in Middle-earth for another 3000 years or so.
The Elves fading away is sort of inspired by Tolkien. By the time The Lord of the Rings begins, Elves have started feeling themselves diminish and are migrating across the sea to Valinor where the effects will stop. But their fading wasn't caused by a lack of light-juice or anything quite so tangible. Tolkien's idea was that as the Second and Third Ages progressed, the era of Elves would draw to an end and the time of Men would begin. The "fading" was a natural transition from one race to another, but going west would ease the Elves' weariness. Tolkien never suggests that sticking two lumps of mithril into an elf's ears would allow them to remain in Middle-earth.
Every Mystery The Rings Of Power's Mithril Retcon Explains
The Rings of Power's mithril-ology may be wild, and may deviate massively from established continuity, but Gil-galad's story of the elf and the Balrog makes more sense than you might think at first glance. Mithril has always been found exclusively in Moria, and The Rings of Power's freak origin would explain why the mineral is limited to a single location in Middle-earth. We can perhaps also assume that the Balrog who struck the tree with darkness is Durin's Bane, and might've descended into Moria wanting to vanquish the Silmaril light trapped within mithril.
Tolkien wrote that Gil-galad's Elves heard rumors of mithril's discovery and established Eregion with the intention of trading with the nearby Dwarves of Moria. Though these events are reinterpreted for The Rings of Power, the Elves' myth about a Silmaril being struck by lightning on top of the Misty Mountains might for how those "rumors" started. Finally, audiences can also rest easy knowing the reason Durin ultimately gave Elves access to mithril after initially going to great lengths to keep the discovery secret. The Elves' very existence depended on mithril, so either the Dwarves weren't callous enough to let an entire race die, or (more likely) Durin and his people realized they could quite literally charge the Elves anything for their mithril and get away with it.
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The Lord of the Rings: The Rings of Power continues Thursday/Friday on Prime Video.