Revealed: A Lost Spire of the Netheril!
Hook
The Arcane Brotherhood's Master Illusionist, Dzaan, born in Thay, and pursuing ancient lore and artifacts in the Frozenfar with Vellynne Harpell, had found something on (or under) the ice east of Kelvin's Cairn.
The wizard had hired a gnome troupe, the kindly-and-popular Gnoobz, heroes of Lonelywood and Termalaine to assist him in a mysterious expedition from which only he and the artisan Spellix Romwod (barely) survived. While the account of events from Spellix have told some of the tale, and inquiries of the dead by Dex other parts, Dzaan succeeded in taking anything further that might have been known to his grave, for he ultimately was burned alive for his hand in the deaths of the troupe. And nothing helpful has been gained through interrogations of his spirit by both Dex and Vellynne Harpell. (Which can no longer be conducted).
The most promising remaining lead is the item Spellix crafted under Dzaan's instruction - a compass made with a needle of Chardalyn.
Result
(See: Spare the Bear!)
Following the compass, and evading threats from various forces seeking the ruin's location (including Zhentarim-backed forces of the Chardalyn Syndicate from Targos) escorted by a giant polar bear (name pending) rescued from goblins and Frost Druid Children of Auril, the site is discovered within a dome of projected illusion - and determined to be what was once a tower spire of The Netheril enclave Ythryn.
Embedded upside down, like a dagger stabbed into the ice, the place had been abandoned for ages upon ages, housing lost lore, secrets of spellcraft, and crafty contraptions. But most surprisingly for the Sewer-side Squad, a greeting by a very much not dead Dzaan (and his erstwhile companion bodyguard wight warrior Krintas - as is customary of the Red Wizards).
This Dzaan was entirely ignorant of the fate of the Dzaan found in Dex's travel pack, and showed grave concern at the world so turning against him. Noting the necromancer's dedication to carrying his bones - and the group's cooperation with his partner Vellynne Harpell, he confessed to his identity as a Simulacrum created by the now-deceased Master Illusionist. He also offered to share with the party his many discoveries within this ruin, which is taken up with rabid enthusiasm by the typically reserved but Netheril-obsessed scholar Alyks Askaria. Giving the man a chance, since Vellynne spoke well of him and his genuine intent to choose a better path for whatever fate might have in store for him - a cautious friendship is formed. Krintas the wight, now loyal to the simulacrum with his master's death, even sharing a dark and dour sort of humor with Sudon.
One being an illusion composed of snow - the other undead, the two were able to subsist without rations for the months left on their own in this place, heeding Dzaan's orders to remain until his return. The party, on the other hand, would need food during their stay - which became the duty of Phantom and the bear, hunting on the ice and carefully watching for additional expeditions and other threats.
This Dzaan, aided by Krintas, offers up his perspective on what happened with the Gnoobz: Dzaan was paranoid about anyone learning about his discovery - he really needed this "win" to gain status with the Arcane Brotherhood. So he had the gnome adventurers remain outside while he and Krintas entered the ruin. Inside, he created the Simulacrum and instructed it and Krintas to remain within the ruin and guard it while he would leave to get Vellynne and other needed loyalist protections to properly secure and study the place. Unfortunately, something activated the Shield Guardian which punched through the chambers in pursuit of a now rapidly-fleeing Dzaan (while ignoring the illusion and the undead perhaps unconcerned in its "programming") - and of course, the escaping Dzaan led it right to the Gnoobz - who also tried to flee - until realizing it was faster and they had to fight it (this happened out beyond the illusory camouflage). All were killed but Spellix and Dzaan, who managed to cause it to malfunction before they themselves passed out from wounds and exhaustion - only to wake being pulled, along with the now broken Shield Guardian, by goblins to Karkolohk. (See: Hostage King).
(For more on the events within the ruin, see In-Spired.)