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Completing the quest will make the NPC neutral.
The events in the quest are set 2 years in the past. Beron has since found other uses for the adventurers.
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Click here for a floor plan of the Common Room
Also, there is some sort of presence that you can feel within the vault - the Genius Loci
A transcript of the quest
Beron: "A troop of Covenant soldiers happened upon the Keep. They attacked, thinking we were a military outpost. We tried to fend them off, but they fought past our defenses and managed to enter the vault. They smashed a relic, and unleashed the curse."
Vestige: "What is this curse?"
Beron: "It's like a disease that weakens and saps your life force. It's fast, and it's deadly. And you don't just die. Your soul comes back as a dire apparition. That's foul, ancient magic."
Vestige: "This isn't a military outpost?"
Beron: "We used our spellcasting might to hold our own against the Daggers. But if they had not released the curse, odds are we would all be dead."
Vestige: "Why are the Telvanni here?"
Beron: "House Telvanni hasn't joined the Ebonheart Pact, but we still have responsibilities in central Morrowind.
We've kept watch over the reliquary under Heimlyn Keep for generations."
Merarii: "Opening the reliquary must have awakened Keeper Vildras, its guardian spirit. Vildras will know where this curse came from and how to stop it."
Vestige: "What is the reliquary again?"
Merarii: "It's an ancient vault built to hold dangerous magical items. This one contains a hidden room, where the most dangerous objects were kept. No doubt that's where the Daggers released the curse. Keeper Vildras can lead you there. Good luck."
Vildras: "You shouldn't be here. A terrible curse afflicts these halls."
Vestige: "I'm here to help"
Vildras: "Then I welcome you, Telvanni-chosen. The spellplague emanates from the hidden chamber. Normally a portal provides access, but the curse has disrupted it."
Vestige: "How do we open the portal again?"
Vildras: "We must force the dead mages who created this curse to help us. Crack open the spell cages holding Rendrasa Malnea and Adras Velae. Summon them, fight them, bind them. Then find me."
Vestige: "Can you tell me the tale of the curse?"
Vildras: "Indeed. Long ago, when our people were still Chimer, this land was known as Veloth.
The Chimer revered the Daedra, but they despised the agnostic class of Elves called Dwemer. The Chimer and Dwemer clashed again and again.
Terrible weapons were made and used. The spellplague was one such weapon. It was created by two mendicant mages who worshipped the Daedric Prince Namira.
This curse was terrible, and worse, uncontrollable. The Chimer lords bound the curse in a hidden vault, then slew Rendrasa Malea and Adras Velae and imprisoned them with their wicked creation."