To reiterate/elaborate on what I mentioned in my earlier post: Jahen has silver eyes because they are a marker of his star elf heritage, which has been kept a secret from both himself and his brother, Ianvic. Their father is a human man while their mother has full elven parentage - albeit half-wood-elf and half-star-elf - whereas her sons both believe themselves to be half-human and half-wood-elf. The truth of the matter is they are each half-human, one-quarter-wood-elf, and one-quarter-star elf. Jahen is the only one between the two of them who inherited significant markers of this, but he also inherited an overall more human-like build (especially compared to Ianvic, who is much more lithe and has never grown a beard), so Jahen has never suspected that his eye color is related to his elven heritage.

Their father, Kovan, does know his past lover has a mixed heritage, but he's not educated on the intricacies of Ruar-tel-quessir history, and doesn't understand the deeper importance to that. She was the one who asked him to keep it secret from their sons, and he agreed to respect her wishes on the matter. She has never involved herself with her sons' lives in any way, which admittedly might be born of selfishness, but it is also partly a genuine belief that the two of them are better off living free lives untethered to her own complicated heritage.

The Sildëyuir, the Yuirwood, and Star-Wood-Elf Lineage

Now, the reason that star elven history is not well known in much of Faerûn is that the star elves had historically lived in a realm of their own making. The "Sildëyuir" was once a region of the Feywild in ancient history - but about 2100 years ago (prior to the start of bg3), the star elves made use of complex rituals and spellwork to break the Sildëyuir off into a demi-plane - it remained attuned to Feywild magics, but became sequestered enough that the star elves could guard it as their home from outsiders. Travel from elsewhere in the Feywild to Sildëyuir remained possible with simple portals, and the Sildëyuir also came to share an extradimensional border with the Yuirwood, which was an ancient forest located back on the continent of Faerûn within the peninsula of Aglarond.

The reason, of course, that the star elves endeavored to draw the borders of their demi-plane alongside this forest is that the Yuirwood was originally their home. In fact, the greatest of the nations of star elves were called the Yuir, and it was for them that this forest was named. Their ancestors were the Yuireshanyaar - a unified nation of star elves and "wild elves" (predecessors to wood elves) that lived in peace for over 5000 years. It was human settlements encroaching from other parts of Aglarond that caused the forest's boundaries to recede and the native lands of the Yuir to come under threat. The Yuir gradually retreated into the Feywild after many centuries of lost battles against humans and orcs, and the parts of the Yuirwood they'd abandoned eventually came to be populated primarily by wood elves.

Over the course of the 2000 years before the Spellplague, with travel between the Sildëyuir (populated mainly by star elves), and the Yuirwood (populated mainly by wood elves), being relatively seamless for even the most novice spellcasters, the two races naturally began to intermarry and have children with one another. The population of star-wood-elves in both regions is significant, and the vast majority of what remains of star elf culture is preserved in these lineages. In time, the star-wood-elves began to wander to other places across the continent, and a significant portion of them settled in the areas around the city of Berdusk. When the Sildëyuir was rejoined with the Feywild during the Spellplague, the event set off a greater migration, as many star-wood-elves in the Yuirwood abruptly lost contact with their sister nation and began to seek after other methods to travel between realms once more.

Jahen's Family

Jahen's mother is a direct descendant of the royal line of star-wood-elves stretching all the way back to the Yuireshanyaar, and she met Kovan in Berdusk. Her last name is Shanyaar, evidence of her lineage. (This would also be Jahen's and Ianvic's last name, which I made mention of as early as this post, never quite sure I'd ever get to give this level of exposition on it. c; )

For anyone who doesn't know much about Berdusk, it's the city where the faction of the "Harpers" originated, and in my general backstory notes for these characters, Kovan is a disgraced-Harper-turned-mercenary who was hired by House Calanthal (the noble house that Jahen's mother married into) to train his would-be lover in the art of swordsmanship. Their affair was mostly rebellion on her part; her noble marriage was politically motivated and at the time her spouse was not holding up his end of the bargain, with the excuse that she needed to produce a viable heir with him before she was afforded the general powers that came with the influence of the family. they were struggling to conceive, and he blamed it on her, and she decided to have children with another man to prove to herself that it wasn't her fault lol.

She ended up enjoying Kovan so much that the affair lasted much longer than either of them had initially planned to indulge it. She is a powerful illusionist and was able to conceal both pregnancies, especially as her Calanthal spouse grew more and more distant from her. However, her second pregnancy (Jahen) did raise a few suspicions among the staff, because she took sudden, unexplained illness a few times in late term. When Jahen was born and she took him to Kovan, she explained that the affair must now end for the safety of all four of them, wished him farewell, asked him to keep her secrets, and gave him a large sum of money to offset the financial burden of raising their sons. Mere months later, her Calanthal spouse died for reasons entirely unrelated to her, and she rose to the power of the role she assumed she had merely married into.

Kovan, whose family was originally from Baldur's Gate, had taken Ianvic back there to be raised far from his mother's political escapades, and then did the same with Jahen when he also came into the world. For all his flaws as a caregiver (admittedly, he was a terrible father and never tried to be better), Kovan kept three unfailing promises to his lover: he never raised a hand to either of the boys, he never breathed a word of her secrets to another soul, and he never spent a single copper of the money she had given him for their sons on any of his own purposes. He used it solely for their benefit to feed, clothe, and house the family for the duration of the boys' childhoods. Whatever was left over when Jahen left Baldur's Gate, Kovan merely gave to Ianvic by way of handoff of a small chest. (probably ignorantly assumed it was the proper thing to do since Ianvic is the "elder" son. However, Ianvic feels very awkward that he was given an "inheritance" of sorts without anything being kept in reserve for Jahen, and has no idea how to broach this topic with his younger brother. This is the primary reason why Jahen has found his letters to Ianvic often go unanswered, and he has no idea of the real cause; he fully believes Ianvic simply stopped caring about him.)

I've played with ideas around their lonesome family dynamics in some depth, and eventually arrived at the conclusion that Kovan has never felt "love" for anyone but the mother of his sons. His feelings for his sons essentially boiled down to "I am responsible for them," and he let go of even that once the both of them had entered adulthood. This could potentially change, but it would be very difficult for him to be open to that change. I do not think he realizes how much either of his sons wished for his approval or affection for most of their lives. Maybe if he did realize this, that would be the key needed to open his mind to change. But therein lies the difficulty - both Ianvic and Jahen learned their relentless repression from Kovan. Admitting their desire for the love of their father to their father was always going to a losing battle from the start.

Jahen's Fevers and the Necrotic Mists

If you've read my bg3 fanfiction (which I write at an absolutely glacial pace lol), you might remember that I've reimagined the "fever" the PC character experiences during act 1 prior to the dream visitor's arrival as something unique to Jahen, rather than something that actually indicates the bg3 party is beginning their change into mindflayers.

This is just something that makes more sense and is more interesting to me for Jahen's story, considering the way the final release of the game indicates only your character experiencing distress during that night (though I know it was different in EA), as well as implying Lae'zel is merely jumping the gun because of her own fears about the transformation. Not to mention the fact that notably, none of the other cult characters infected with the netheril parasites throughout the rest of the story ever show even the barest signs that ceremorphosis is liable to trigger randomly, so to imagine that the party is starting to change merely to lend weight to the plot is a difficult leap in logic for me.

However, I do think the characters themselves being primed for that leap in logic is an excellent basis for an episode of Jahen's chronic fevers to give them a scare and prompt the dream visitor to make their first move, while also giving Jahen a fantastic reason to doubt his own reality early on and make him more vulnerable to the Emperor's attempts at manipulation. So Jahen having chronic fever-driven episodes was pretty much always a part of my story development. I think I worked in the star elven heritage to this about a month before I finished the final act of the game.

Calling back now to Sildëyuir as the realm of the star elves - one of the main driving forces behind why star elves kept crossing the extradimensional borders of the Yuirwood to live and procreate with the wood elves is because there was a slow invasion happening back home in Sildëyuir, carried out by extraplanar beings known as the Nilshai. The Nilshai were aberrations who were highly intelligent and exceptionally skilled sorcerers who developed a perverse obsession with the star elves in the course of their general pursuits "to conquer new realms and planes for their own alien kind." Here are some pieced-together excerpts below from the forgotten realms wiki:

"[The Nilshai] opened several dark portals through the dimensions to access [Sildëyuir], in order to invade and twist it to their own strange ends, by altering the plane's very essence. Each invasion was harder to beat back than the last, and by 1372 DR, great stretches of Sildëyuir had been corrupted. [The Nilshai's magic] created sluggish, silver-gray mists, marking the border of a creeping blight. Beyond them, Sildëyuir is tainted; the trees are dead and rotting, and thick, oily black moss grows everywhere. The silver starlight in these areas is gone, and the sky is black; the air is humid and stinks of overpowering rot. [...] The Nilshai now occupy citadels that were recently occupied by star elves, the former residents mysteriously vanishing. Some star elves even called for their race to return to the Yuirwood and abandon their realm to the Nilshai, while others wanted to seek allies instead."

These magics, as designed by the Nilshai to slowly eradicate the nation of the star elves, were most deeply attuned to affect the royal bloodline of Yuir, who were the guardians of the star elves' vast collection of King's Tears, which the Nilshai greatly coveted. The cursed blight could incapacitate any elf in direct doses, but even thin traces of the same magic would strike down the royal bloodline with terrible fever and slow, painful death. This enabled the Nilshai to make captures from the ruling lineage and potentially torture them for information on their repositories of knowledge wherein their most prized King's Tears were kept.

The necrotic mists of the Nilshai originally were confined to Sildëyuir and didn't cross over into the Material Plane of their own accord. But shortly before the Spellplague began, a corrupt human sorcerer by the name of Carthoun Misintle began acting on his ambitions to take power on the northern border of the Sea of Stars. He discreetly gathered a collection of magical items to aid him in concealing his evil nature and the true extent of his power until an opportunity opened for him to seize his selected throne under a magical guise. In order to build his power base and his collection of artifacts, he studied a variety of evil magics before beginning his voyage north. One such study he conducted by traveling through the portals of the Yuirwood into Sildëyuir and collecting samples of the Nilshai's gray mists. The necrotic properties of the mists were greatly appealing to him, as his greatest goal was to pursue lichdom, and he frequently animated undead servants to carry out his wishes.

When Misintle made his return to Faerûn, he carelessly released large quantities of the necrotic mists into the world so that he could continue to study them over long stretches of time. He discovered that exposing the mists to any sort of necrotic magic would cause them to grow and swell, while also noting that the star-wood-elves of the Yuirwood seemed to have managed to breed out their intense vulnerabilities to the mists. Eventually, he abandoned his experiments and continued his conquests with naught but a few bottles of necrotic mist in his arsenal, and left the rest behind to dissipate.

The mists however, have sustained themselves for over a hundred years now in the Material plane, feeding off any corrupted bed of rot or coalescing around sources of necrotic magic. The wood-elf side of most star-wood-elf family lineages helps balance the star elf vulnerabilities to the magic and prevents extreme illness. However, Jahen - and possibly a few others out there in the world - inherited vulnerable traits from his star-elf parentage, while his human half diluted out his wood-elf genes juuust enough to make him susceptible to the mists again. And because he traces his heritage back to the royal bloodline, he is exceptionally sensitive. Even trace amounts of the mists will induce unexplained fevers in him that he can do nothing to cure or alleviate.

Example sources of unnatural rot in Faerûn that attract the Nilshai's gray mists:

Food that is stolen from the hungry and left to rot (present in the Undercity, which is what led to most of Jahen's childhood fevers, as well as in the goblin’s camp in act 1 of bg3, which is the exposure that induced his act 1 fever)

Blood and viscera left on weapons from unjust murders and left to rust (present near the outskirts of towns that Jahen once roamed as a solo ranger.)

Corpses desecrated by magic or profane touch and left unburied rotting aboveground (Mystic Carrion NPC heavily involved in this. induced jahen’s fever in act 3 fic)

However, ordinary necrotic magic unconnected to the mists is unlikely to present any unusual symptoms in Jahen's condition. And I'll reference Shar's magic in the Shadowcursed lands as well - it’s technically divine magic and cannot be “corrupt” in this sense, but it does heavily affect him emotionally and wears on his spirit over time. The mists being unable to cross planes without intervention from magic users is also why Jahen does not experience any episodes while he's in Avernus with Wyll and Karlach; despite all the other corrupt magic there, there is no presence of the specific necrosis designed to target his ancestors.

Because so much of this history and worldbuilding plays out around the nation of Aglarond, and I also ended up working in large chunks of Aglarond lore to my plan of giving the sun back to Astarion, I've essentially committed to the idea that if I ever do getting around to writing that, it'll end up being a dual-plot fic with Jahen's history on one side of the coin and Astarion's future on the other. But I haven't nailed anything down in specifics yet, always reading more to learn the ins-and-outs of as much lore as I can. :] thank you so much for reading if you got this far! <3 feel free to ask me any questions you might have.

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Sources and resources:

Yuireshanyaar, Yuirwood, and Sildëyuir - Forgotten Realms Wiki

Map of Faerûn - by user Divinity_CV on reddit

Introduction to Berdusk - video essay by Ed Greenwood