"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.