Tes5Mod:The Forgotten City/The Forgotten City (quest)

Quick Walkthrough
#Travel to The Forgotten City.
#Speak to Cassia.
#Enter the Forgotten City
#Claim the Lakehouse key

Detailed Walkthrough
Upon reaching level 5, the Courier will approach you, saying "I have a letter here for you."

This will begin the quest, and Cassia's Plea will open and read itself to you. The note invites you to investigate the promise of "Riches Beyond Imagining", which Cassia promises can be found in Forgotten Ruins in The Reach. She will request that you travel to the ruins and help her find her brother, Altrius, who explored them but did not return.

The Forgotten Ruins are just west of Purewater Run and east-south-east of Hag Rock Redoubt, between two waterfalls. After entering it behind the first, its interior door will automatically open for you, revealing Cassia standing before a pedestal and a plaque written in Dwarven lettering: "Riches Beyond Imagining".

Approach her, but be ready for her to draw her sword as she angrily yells ''"That's close enough who are you?" Depending on receipt of her letter, following your introduction, the conversation could proceed one of two ways:

:Who are you?
::"What do you mean? Didn't you get my letter? I paid good coin to a courier to track you down, and you're saying you just came here by coincidence?
::Well, perhaps it's fate. The thing is, my younger brother went down this shaft a few days ago and I'm worried he's stuck down there, or worse.
::I know this is a lot to ask, but I need you to go in after him, and bring him back to me alive. Do you think you could do that for me?"
:I got your letter
::"Oh, it's you! I'm so glad you're here. Look, I don't know how much time we have left. My brother went down this shaft a few days ago and I'm worried.
::I need you to go in after him, and bring him back to me alive. It could be dangerous, but if anyone can do it, it's you. Now, are you ready?"
:I'm ready.
::"I knew I could count on you. Now quickly, go!"
:Not yet. I have some questions.
::"Alright, but please hurry."
::Who are you?
::"My name's Cassia. My brother and I came to Skyrim from Cyrodiil with the Imperial Legion, but we took a... break... to explore these ruins."
:::So you're both deserters, then?
:::"We didn't want to get involved in this stupid war in the first place! We only came here because we were conscripted.
:::But our unit was wiped out by Nords and those Forsworn savages, and we saw a chance to get away from the fighting altogether. So we took it."
::What happened here, exactly?
:::"Well we came here hoping to find something valuable, to pay for carriage back to Cyrodiil. Altrius went down the shaft, but he asked me to stay here."
::Why haven't you gone down there?
:::"Because he made me promise I'd stay here and wait for him, no matter what. And I keep my promises. Well, except as far as the Legion is concerned."
::Why did you send for me, of all people?
:::"I was in Markarth looking for help, when I heard people talking about you, and all the loot you've been trading with merchants all over Skyrim.
:::I figured you'd be interested in exploring these ruins, so I asked a courier to track you down. And here you are."
::That's all the questions I had.
:::"So you're ready to go now?"
:I didn't come here to help you 
:"Damn you! I knew sending for you was a mistake! Get away from me!"''

At this point, you can inspect the plaque in front of her, or read the Dusty Journal on the small pedestal. Once you have concluded your business on the surface, jump into the unknown. Walk through the passage in the ruins, and enter The Forgotten City. Once inside, you will find an abandoned township under the ground, with no life at all save for bats and skeevers. The area is strewn with burnt corpses, and the atmosphere is one of abandonment. As you approach a dead tree, a hanged Old man drops from his noose in front of you. Beside his corpse lies an empty wine bottle, quill, and a Suicide Note. The note proves that the body is Altrius's; it intimates his tale of infinitely entering and re-entering the Lakehouse. His note goes on to urge you to kill yourself rather than endure the same fate as he did. If you choose to pursue this course of action, you may drown yourself in the lake. Otherwise, enter the Citadel to obtain the key from Altrius's ghost. As you enter it, his voice will alight the passageway and tell you that "I was in your shoes once, a lifetime ago. I remember the way the lakehouse calls to you."
As you ascend the stairs to the Citadel's balcony, the ghost continues his story: "I thought to myself: Do I want to die alone in here, or do I want to open that door?"
:"It seems like a simple choice. But in the end, death was the better option all along."
:"Please understand. I locked the Lakehouse door to stop you from making the same mistake I made."
:"And now you ignore my words and come searching for the key."
After the final stairs and the final warning, you will see Altrius's Ghost around a corner. As you round the corner, it will charge from behind its own at you, yelling "Stay away! I hid that key for a reason!" Battle and defeat the ghost. Once this is complete, exit the Citadel to its balcony, on which a small chest sits. Open it to take the key, but be prepared for a surprise: The balcony will drop from beneath you, and a voice will whisper into your ear: "Lakehouse." After you have recovered from the significant fall damage, head to the Lakehouse and enter it. Once inside, do not try to leave, as a voice will angrily yell "My letter!" each time you attempt escape. Instead, go to the desk and read Metellus's Note. The letter charges you with a mission: Prevent the desolation of the Forgotten City. Once Metellus's last words have been spoken, you are free to exit. Do so, and the door will reveal a strange portal that will cast you into a different time: The Forgotten City will be bustling, illuminated by the sun under the ground. A new quest, Forget-Me-Not will begin.

Quest Stages