"So you're saying that standing stone we passed is what the years are counting from?"

"How do you not know this?"

"There's a song about it, have you heard that one? O Standing Stone, O Standing Stone, Monument to our new home..."

"Never had any lyrics to that tune back in my day."

"Please keep your voices down. Pass Without Trace isn't any help if you're not even trying to be stealthy."

"Lumi? Cease travel where we might converse," comes the telepathic voice of her familiar. "Our prospects on this quest are growing worse."

"Hold on, Scarloc's coming with bad news," Lumi whispers. "Any good hiding-places nearby?"

"Let's try that Netherese tower-fragment. There's enough still standing we can hide inside, but not too much that we can't check it out for anything already lurking there from a distance."

The familiar reappears, dismissing their innate invisibility. The sprite hovers before the party at parade rest, wings buzzing faintly. 

"First, magic up ahead has gone awry. Odd warping even to my untrained eye."

Ivaebhin pinches the bridge of his nose in recollection, then retrieves a reference book from his pack. (Elminster's Ecologies, vol. 1, Lyra Sunrose.) 

"That would match the reported energy drifts from the mythal, but those should only be present in the edgelands, on the forest borders, not here in the city. Such zones are intermittent, but on a scale of seasons."

"Is it dangerous?"

"They can negate magic cast in close proximity, which would impede, oh, every one of our number. There's also some accounts of them affecting the appetites of animals. Snakes choked to death on apples, bloodthirsty squirrels, and so on."

"I don't think I need a spell to handle a squirrel."

"You might for a whole tree full of them swarming you. We have no Alchemist's Fire, last I checked."

"Point."

"In theory, we could undo the warping ourselves rather than wait for a natural dissipation. As a manifestation of a twist in the Weave, the cause could be addressed by righting the imbalance, casting a spell of the appropriate school to release the tension..." 

"Second, there's more than squirrels to avoid. I saw three gray-skinned mouthless humanoids. Invisibility concealed me not; their eyes tracked me as I flew far aloft."

"Sounds like doppelgangers. Can't hide from thought-reading. We'd better circle around them."

"Then I'll resume my duties as a scout."

The landscape is scattered with broken masonry amidst the tall trees, giving an eerie whistle to the cold wind blowing through, and a huge mound of rubble stands a mile to the south. True to the Yeoman's report, strange eddies of energy are dotted around the scene, leaking and pulling the energies of the Weave in a way that distorts light like a heat haze. 

"Evocation."

He takes out a forked twig, scorched to charcoal. 

"Ivaemaskan."

A crackling arc.of lightning streaks from the twig to the center of the nearest distortion, almost thirty feet away. The next moment, a hush ripples out through the air as both the Witch Bolt and its target disappear. 

"Stay clear of the nodes if you can, within fifty feet the draw is so strong you have to cast something and it'll be wasted if not the right school."

"That can wait, Scarloc says the doppelgangers are coming over. They'll fly interference, buy us some time, they say."

"Already? Hells."

The two of them hustle back to Mitrol, balancing on the trunks of fallen trees to avoid tripping noisily on the scree and roots.

Their druid waves as they get near, suddenly appearing from the haze of concealment of his spell that made him indistinguishable from the tree trunk behind him. 

"Which way?"

Lumi tosses a dagger up, and it lands pointing south. "That way. Any way but the way we came would do, Scarloc thinks we're headed back to the broken tower so the the doppelgangers will think so too."

"And what if your sprite leads them to our true destination in ignorance? You'd better summon them back once we're hiding."

"Doesn't matter, if they catch us here first!" Fala says, also appearing. "C'mon, speed over stealth now, Mitrol'll clear the tracks."

She takes the lead towards the rubble-pile to the south, running up the sides of trees with an uncanny adhesion and swinging off branches with her whip for the height to guide their route ahead between more of the magical disturbances.

From behind the group comes another stilling of the air, that strange release of tension, then another. 

"They have a spellcaster, and it's figured out the trick."

In the magical quiet, a voice can be heard, distant but clear, singing a wordless elven melody from beneath the mound of wreckage. From further out echo demonic calls, cackles and hooting Abyssal cries.

The rubble rises a quarter mile into the sky and stretches nearly as wide along the ground, a mixture of elven and Netherese structures crushed together. Each time another of the disturbances in the Weave is eased by their pursuers, the remaining tangles flare and dance wildly, the singing becomes a little louder, and the colossal pile of rubble shifts a bit.

After a few such instances, it becomes apparent that the shifting is from the elven masonry moving back together, missing pieces replaced by the darker Netherese stonework. A tunnel becomes visible at the base of it.

"Inside!"

"Is it stable?"

"Do you have a better idea?"

Despite the destruction outside, the tunnel is in fact in good repair. What little debris or clutter is present is migrating outside with each pulse of silence. 

Once everyone has made it forty feet in, Mitrol drops his emanation of concealment and casts a new spell. The stone of the tunnel behind them extrudes spikes and thorny barbs, like the stalagmites and stalagtites of a natural cave or the tongue of a cat. The next moment, the colors and shadows shift to hide the hazardous terrain.

"That gives us ten minutes, unless one of them can teleport."

"They have been using a lot of spells... Or else they've found a way to use cantrips for the untangling."

Ahead, a bronze placard is mounted above the doorway. In Elven, it reads:

Entering the arcane sanctum of Quaryl Tellasarim, lorenorn of Myth Drannor. Enter this place and learn, oh seekers of truth. May Mystryl guide your way.

"A lorenorn! We must have the right place. A sworn protector and tutor of our people will see those doppelgangers off easily."

"...kessuk! Doppelgangers, we need to worry about impersonation and replacement." 

He thinks for a moment. "No, Mitrol is likely clear, a doppelganger druid would be very unlikely. You had eyes on Fala the whole time? That leaves myself and Lumi. Lumi can resummon her sprite, but I don't have any spells that another wizard couldn't copy."

"Moonbeam exists."

"As a last resort! I'm not some freak of nature like Fala, a few moments in that would kill me."

"Scarloc can use their Heart Sight. Speaking of."

The sprite appears with a faint pop, this time from being dismissed and resummoned instead of ending their own invisibility.

"His heart's of law alone, as was he last. The monsters are outside, we should move fast. My warning shot struck not their hearts afear, once through the warps they will be coming here."

They vanish again to flit ahead as the group's scout. The others follow to stick within the 60' range of Lumi's telepathic link to her familiar.

"Why aren't I Good? I do Good things all the time," Ivaebhin mutters under his breath. 

"Another one of the things up ahead, says Scarloc. Room's full of books and floating glittery shields."

"Abjuration. I have Mage Armor prepared, but it's touch, it might not work so close... No, I can cast it at a distance and then deliver it up close, nevermind."

He does so, following the sprite into the room and the pulled thread of the Weave is restored.

"One more of those, then two more of my second-level slots, and then I shall need a break to study my spellbook."

He takes one of the books from the shelves, opening it. "Property of the lorenorn."

"Shouldn't you be worrying about curses and checking everything first?" Fala asks. "I have a distinct memory of a lecture on that topic when we found that chest."

"This is a place of learning dedicated to Mystryl, she may be dead but to bar others from magical knowledge would be anathema to her. Besides, a lorenorn owner is all the protection these books could ever need."

He keeps reading. "This whole room is dedicated to the study of Abjuration, the pulled thread may have emerged here for that reason. Mythals are predominantly built around Abjurations, and this tome deals with the role of that spell school in the creation of intelligent items which ties in with the sentient mythal hypothesis..."

"There's another knot for you to deal with in the next room. It's a creepy one too."

"What kind of creepy?"

"Burnt bone smell, gray fog so thick you can't see more than five feet ahead -"

"So that's the Necromancy room, with a Necromancy thread to be fixed. Anyone got a spell for that?"

"Nope, okay, ask your Yeoman to come back with as many books as they can carry and dump 'em next to Ivaebhin. We'll try the other route."

"Next room is Conjuration, going by the books. Not sure what clockwork toys have to do with it."

Goodberry. Strange to be casting it on a magical knot floating in space instead of a sprig of mistletoe, but it seems to work. 

The Enchantment room is lined by mirrors -

"Sleep." His face is still buried in a book, this one on Thayan life-draining rituals. 

And then the room after is one of Illusion, spectral elves emerging from the walls to drift across the space in a ghostly dance while another ancient elf with atrophied and translucent flesh stands motionless in the center, as though lost in thought or befuddled by the figments. 

Scarloc resolves the thread in that room with their own innate Invisibility.

"This must be an image of the lorenorn!"

"Why does he look halfway to a skeleton?"

"It's complicated."

"Is this sort of Illusion supposed to be... tactile?" Fala asks. She withdraws her poking finger like it's been burnt. 

"Illusions are not reality, the map is not the territory, no, but that is not to diminish the effect that they can have on that which they represent. When soldiers march it is the lines on the map that dictate their movements as much as the reality of the terrain, and though territory is an abstract concept without physical grounding it is ignored at one's peril," the figure speaks.

And then he vanishes, not with the abrupt absence of an Illusion ending or an Invisibility starting but with the faint pop of a teleportation.

"...I swear by all the Seldarine, if that was one of the last surviving lorenorns and you poked him -"

"You said he was an illusion!"

"He didn't seem insulted by it. Let's continue, if we see him again we can warn him about the doppelganger mages."

Transmutation: Prestidigitation. 

"Cantrips work, just need to resist the pull to use a levelled spell."

The floor here feels like putty underfoot.

Divination: True Strike.

This one of the octagonal chambers is lined with viewpoints, projecting the sights of dragons soaring and storms roiling. 

Evocation: Booming Blade.

Just when Ivaebhin is thinking he's getting a good grade in showing off his spell coverage to the lorenorn, the scorched room shakes underfoot, the whole complex reconfiguring. 

As the party variously shouts, dives for cover, or goes to the doorways, the tumult subsides again. Where there once was a wall, an extradimensional space has opened up, like the mouth of an open Bag of Holding. 

Inside is a comfortably appointed living space where the lorenorn awaits. He beckons the party to enter.

"Please forgive my companions their hesitance, it's truly an honor to meet you. Quaryl Tellasarim, I presume? I am Ivaebhin Aelorothi, of the Evermeet Aelorothis."

"A pleasure. I have been trapped here for some time, ever since the city fell, with little company."

Also inside his chamber, not visible from the entrance, are two statues of mummified mind flayers. 

"There's so much we should ask you, we're fellow seekers of truth, trying to work out whether the perpetual winter over Icewind Dale is the work of a misused or malformed mythal - but first, are you aware of the magical edgelands anomalies all over the area? Or the doppelganger mages approaching?"

"Learning is the most valuable reward." He waves a hand, and a few books drift over towards the party. "These discuss the weather-regulating aspects of Myth Drannor, may they grant you insight."

"As for the pulled threads, they exist as a consequence of some invasive research I have been performing, on matters of arcane permanence. The Weave will repair itself in time; Mystryl knows what is occurring here. Those threads will be snipped or sewn back in, and those that are trimming the burrs from the Weave will be richly rewarded."

"Your pursuers are at the entrance." He chuckles. "Not doppelgangers but ultroloths. An understandable confusion, perhaps the batrachi cultivated their spies and assassins from that fiendish stock. Please excuse me for a moment."

He vanishes with another teleport. 

"Those were ultroloths? Three of the greatest of the Greater Yugoloths? We need to help him somehow, they're inherently magic-resistant but maybe -"

There are a few important considerations that shape high-level combat:

Anyone without a legendary resistance to spells is gambling with their life every time they allow themselves to be targeted by a spell of sixth level or above. Even a fourth or fifth level spell can be decisive if they have backup ready to chain you up while you're Polymorphed into a pig or hack you to pieces while you're Held in place. 

Invisibility and other illusions become irrelevant for concealment and deception within the ranges of special senses such as True Seeing. Physical barriers to line of sight or effect become more important again.

Killing your opponent is often little better than allowing them to escape, if they have a Clone or a phylactery or an arrangement for their resurrection, or are an outsider off their native plane. Flesh to Stone, Imprisonment, and True Polymorph can be preferable for rendering foes harmless for as long as their allies cannot retrieve and free them. These are however high-level options, so a Simulacrum or other decoy can sometimes bait enemies into expending those resources.

In short: Don't be targeted, don't rely on illusions to avoid being targeted, and don't assume death is enough to remove a threat more than temporarily. 

These have remained the case through multiple large-scale shake-ups of how magic works, so even someone who has been living under a fallen city since the last rebirth of Mystra would be aware of them. 

The trio of ultroloths have entered and made their way to the Necromancy chamber.