From the WotC plot recap:
At last, the true enemy of Loudwater was revealed to be the fallen first Green Regent. The corrupted champion of Mielikki sought to return to the Delimbiyr Vale. The key to his victory and defeat were the Heart and Mind of Karsus.
- - Teseryne Truesilver
- - Daughter of Talanthe Truesilver
- - Year of Rogue Dragons
The Order of the Jade Blade passed the rest of the winter in Waterdeep. While they waited for the thaws, Otar, the orcish Green Regent, explained to them the true nature of the threat that faced Loudwater. Centuries ago Mielikki chose a dragon by the name of Guev'Ressunvee to be the first Green Regent of the Delimbiyr Vale. Guev'Ressunvee twisted the powers given to him, becoming a cruel and arrogant tyrant. The Forest Lady stripped him of his mantle and cast him out of the world. Guev'Ressunvee plotted his return for centuries and found a way through the ruins of Karse. Karse was a floating city of Netheril that had fallen from the sky into the southern High Forest when that ancient empire collapsed. The Green King was using the strange, arcane energies of the ruins of Karse to destroy the bindings that kept him out of Faerûn.
Otar said that Mielikki had come to him while he was comatose and explained how to foil Guev'Ressunvee. For the Green King to break his exile, the Coalescence of Scorn needed to be absorbed by the Heart of Karsus. However, if the Coalescence of Scorn were to be absorbed by the Mind of Karsus instead, the fragmented consciousness of Karsus, the archmage who caused the fall of Netheril, would realize the gravity of his sin and find a lasting peace. This would close the path that Guev'Ressunvee planned on using to enter the Realms.
In the spring of the Year of Rogue Dragons, the Jade Blade returned to Loudwater and the Delimbiyr Vale. Otar believed that the Green King would be watching him in particular. So he gave the Coalescence of Scorn to the Jade Blade and asked them to enter the Mind of Karse. Otar would enter the Heart of Karse to block the Guev'Ressunvee from using his foothold there.
Parting ways with Otar, the Jade Blade journeyed to a part of the southern High Forest called the Dire Woods. At the heart of this eldritch woodland was a butte that rose hundreds of feet into the air. After several hours of searching, the adventurers found a game trail that wound up to a strange rock formation on the butte face. At the base of the trail were three large, green crystals jutting out of the forest floor. Our heroes recognized them as the Eyes of Guev. They had seen the same crystals in Undermountain and knew that they allow the Green King to see the area around them and summon allies there, which Guev'Ressunvee promptly did.
A bone devil stepped out of each of the crystals and immediately began to summon more of their kind. The Jade Blade realized that the gems held the devils here. They shattered the crystals, which pulled the devils back to whatever hell they came from.
The adventurers climbed the path, which ended before a twenty-foot tall carving of an ugly face with its eyes and mouth closed and twisted in pain. When the Jade Blade approached, a rumbling voice demanded answers to a series of questions regarding the fate of Karsus. The adventurers knew that his most hated foe was Mystryl and that he loved power more than his own people, which answered two of the questions. They were then asked to name Karsus' final words, which happened to be contained within the Coalescence of Scorn. They placed the orb in the face's mouth and the carving slid aside, revealing a tunnel into the darkness.
The Jade Blade retrieved the Coalescence from the carved face and entered the butte. Shortly afterward, they met a huge, disembodied eye that flew through the air. This was the Neversleeping Eye and was all that remained of Andoris Deranthar, an arcanist who administered justice and would hunt for spies in Karse. The Neversleeping Eye demanded that the adventurers identify what was the greatest thing one could serve. Our heroes deduced that the answer the Eye was looking for was truth. The Eye then demanded to know the adventurers' mission. When they answered truthfully, it allowed them to pass.
The Jade Blade continued to explore the Mind of Karsus and encountered an apparition that told them that they were fools for intruding and that they would now have time to consider their folly alone. The adventurers were each teleported to separate rooms where they fought a variety of monsters. During the combat, they were continually teleported from room to room so that they each fought the different monsters. Once all the monsters were slain, they returned to the main chamber and were able to move on.
Shortly afterward, the adventurers were ambushed by mindflayers serving Guev'Ressunvee. They slew the aberrations and entered a maze made of walls of force. Mist filled the rooms and provided perfect cover for the destrachans, who are sightless and tracked the adventurers with their uncanny hearing.
After defeating the destrachans and finding their way through the maze, the Jade Blade came upon a large cavern. Zacharia, who was once the chief constable of Karse but was now a helmed horror, sat on a throne in the center of the cavern. She questioned the Jade Blade as to why she should grant them an audience with the greatest of all wizards. Our heroes showed her the Coalescence of Scorn, and she agreed to let them pass. Zacharia warned them that the Mind of Karsus was but a fragment of the last moments of the Karse enclave and was haunted by a beholder. Zacharia would not permit the coalescence to enter until the eye tyrant was killed.
The Jade Blade passed through the arch and found themselves in a ghostly recreation of Karse. The city suddenly shook violently and started to fall. Wading through the panicking spectral citizens, the adventurers found the beholder and killed it. Zacharia appeared and showed them where to find the spectral Karsus. When they touched the Coalescence of Scorn to the specter, a look of shock and understanding filled his face, and the orb, Karsus, and the city disappeared. Karsus' mind was finally at rest, closing this portal to the Green King.