Return the Lexicon to Avanchnzel.
Located in Riften is an NPC named From-Deepest-Fathoms, who seems to be deeply troubled by an object she has acquired from Avanchnzel. Saying that the memories of the dead haunt her thoughts, she beseeches you to return it to its proper resting place. Should you accept, you will receive the Lexicon, a strange, ornately-decorated cube as well as the location marked on your map.
As you begin to progress through Avanchnzel, you will begin to see ghostly flashbacks of the previous owners of the cube – a party of adventurers, one of whom is From-Deepest-Depths. As you progress further, you see their paranoia increase as what should have been a simple job begins to turn deadly. Slowly, one by one, you’ll find the corpses of the three party members who did not make it out, until eventually you will arrive at the room containing the altar where the Lexicon is to be returned. It is guarded by a Dwarven Centurion Guardian, so be careful. Other dangers are several Dwemer robots as well as traps. You will approach a spinning blade trap. Just to be safe, you might want to tell your companion to wait until you’re able to deactivate it via the lever.
The dungeon can be left by going to the upmost point through a door leading to the Avanchnzel Balcony.
Once you return the Lexicon to complete the quest, you will receive a permanent “Ancient Knowledge” bonus, lending you 25% bonus armor rating when wearing Dwarven armor, and a 15% bonus to how fast your Blacksmithing skill increases.