"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. They stand before the thick fog filling the room. 

Blink.

Apparate.

The lorenorn appears inside the dense mists, moves to its border, and casts a spell once he can see the corridor containing the intruders.

Sleet Storm.

The air fills with driving precipitation, icy rain too thick to see through that buffets the ultroloths. The floor immediately becomes both slick and covered in little hailstones like ball bearings. All but one of the ultroloths lose their footing, falling prone.

Before they have any chance to respond, Quaryl has retreated back inside the mists of the Necromancy chamber, and then his Blink spell shifts him onto the ethereal plane.

Ultroloths are schemers, masterminds. It is very rare for them to cooperate for any length of time. It is very rare for them to personally engage in combat where minions could be sent to fight for them. 

The three invading this sanctum of elven lore do not immediately flee. They coordinate over telepathy.

One dispels the Sleet Storm, rising up from the ground to levitate with their innate flight. It directs a hypnotic gaze into the mists, the psychic blast not one that needs sight to connect, but its target is not on the Material plane to be struck at this instant.

The second regains its footing, and then lines the corridor with a Wall of Fire. The fiends are all immune to the blazing flames that fill the space.

The third attempts to dispel the mists of the next chamber, only to find them nonmagical or else too powerful to be removed by a single casting. Its mercurial whip coils as though prehensile and weightless as it silently glides into the fog-shrouded room.

Quaryl is visible to the ultroloth's truesight, once it comes close enough to glimpse him through the mists. It has no way to affect him on the ethereal plane, so it moves to his position and readies a flurry of attacks for his return. 

Blink returns him to a position within ten feet of his departure, so he reappears far enough away that the ultroloth cannot see him through the mists. 

Readied Sleet Storm. He has long since passed the point of needing to provide any components for such basic spells.

This time his Blink does not activate.

Apparate.

Appearing at the far end of the corridor by the entrance to his complex, Quaryl releases the spell. Ice wars with fire, and the space is filled by steam as much as sleet.

Before the question of whether a Wall of Fire counts as 'exposed flames' for the purpose of being doused by Sleet Storm can be answered, the ultroloth concentrating on it is knocked over by the torrential downpour and the spell collapses for that reason instead. 

Being knocked over, it can't move close enough for a hypnotic gaze even with its supernatural flight. Irritated, it clears the sleet with another Dispel Magic and then prepares that attack for when the lair's defender comes in range. 

Befuddlement.

Apparate.

Mind blasted, the ultroloth reels. Its peers confer over telepathy at the speed of thought. 

They have no way to restrict the movements of their foe, who knows the lair better than them. Caught exposed, they too might lose their spellcasting for a month. It doesn't take long for them to reach a conclusion.

The three of them Dimension Door away.

"- Bless up, any other minute buffs?"

"There, they won't be troubling us. Where were we?"

He takes a seat.

"Whether something like the mythal of Myth Drannor could be causing the endless winter in Icewind Dale, or whether it's the work of fey from the Unseelie Court."

"The two are not necessarily exclusive. To my knowledge only human wizards have succeeded in replicating mythals, but the fey are known for employing intermediaries -" he casts a significant glance at Lumi "- and being magically potent in their own right. I do have another hypothesis, for you, however..."

One tome on his desk opens with a wave of his hand, the pages turning through a list of different phylacteries employed by liches and onto a diagram of a crystal next to a strange geometric tower of hard edges and sharp angles. 

"Have you ever heard of Cryshal-Tirith? A relic of ancient Zakhara, one that could raise towers of crystal that would absorb the light of the sun to grow like plants and store magical power. Some sources suggest that it ended up in the region you are concerned about. There have also been more recent tales of the earthmotes, rock formations magically suspended in the air. Suppose that Cryshal-Tirith was so elevated, able to drink in the sunlight and steadily spread? Being translucent, the people on the ground could perhaps only observe a perpetual dimming of the sun and consequent cooling, that might be mistaken for a winter that never ends..."

He trails off, lost in thought. 

"... excuse me?"

"Some kind of fugue?"

"I suppose he's told us everything we need to know, so perhaps we should depart?"

Then the lorenorn's features grow slack, his eyes empty, his jaw stretch open grotesquely wide, and flickering dark sigils fade into visibility around him.