Unfathomable Depths

Unfathomable Depths is a side quest in The Elder Scrolls V: Skyrim.

Overview

 * Quest giver: From-Deepest-Fathoms - Riften Docks
 * Reward: Ancient Knowledge (Permanent Bonus - 25% bonus to armor when wearing Dwarven Armor, Smithing skill increases 15% faster).
 * Reputation gain: N/A

Objectives

 * Return the Lexicon to Avanchnzel

Noteable Loot

 * Lots of Dwemer parts which can be smelted.
 * Gems and Soulgems looted from enemies.
 * After you go down the stairs into the 3rd level, where there are 4 Dwarven Spiders, turn to the right and go through two dark doors. Inside the room there is: a full set of Dwarven Armor, Potion of Ultimate Stamina, and a Necklace of Extreme Stamina
 * There are several items in this area that help increase/improve stamina

Walkthrough
The Dragonborn will find From-Deepest-Fathoms wandering around Riften docks. Upon first meeting her, she seems despondent and paranoid, immediately handing the Lexicon to the player and pleading with him / her to return it to the Dwemer ruins of Avanchnzel. Upon arriving at Avanchnzel, four orange figures will be standing at the bottom of the big open room. These are the ghosts of four adventurers, who had been sent to Avanchnzel to recover the Lexicon. They will reappear throughout the quest and consequently, the Dragonborn will find out that the two human characters actually had little knowledge about the place or item of interest, but From-Deepest-Fathoms and the Argonian male were very familiar with both and persuaded them to stay and complete the job against their better judgement. The two human mercenaries stay, only because the job paid well, but it becomes clear that the Argonians sought out the Lexicon for the knowledge it housed.

As the quest progresses, the Dragonborn will find three bodies, from three dead adventurers, meaning only From-Deepest-Fathoms made it out alive. At the end of the dungeon, the Dragonborn will find the Receptacle in which to place the Lexicon. The receptacle for the Lexicon is guarded by a Dwarven Centurion. For a higher-level character, the guard was a Dwarven Centurion Master.

For lower level characters, Dwarven Centurions can be a headache, but there is a simple way to kill them. Once you are close enough and it awakens, shoot it with a couple of arrows and lure it down the hallway that took you to the Lexicon receptacle. Once you pass through the doors leading to the ramp (with the spinning blade trap), re-activate the trap and run to the top, setting the blade in motion. The Centurion will attempt to run through the trap and fail, falling dead without you ever having to land a blow. Alternatively, characters with a high Archery skill plus a powerful bow enchanted with fire can take down a Dwarven Centurion Master in just a couple of shots.

Upon activating the receptacle, the Lexicon will be removed from the player's inventory and open. The Dragonborn will be rewarded with Ancient Knowledge, a permanent bonus. The Quest will also be completed.

This dungeon is not accessible before the quest is started. (Dungeon Lock Outs)

Make sure to loot everything (as always) because many of the Dwemer constructs drop great loot. Also make sure to unlock gates and check safes as they lead to great loot as well. Loot ranges from Flawless Garnet/Amethyst to Grand Soul Gems.

Note

 * From-Deepest-Fathoms is not marked as an essential character so she can be killed if attacked. The Lexicon will still be in her inventory after she dies.
 * Stealing the Lexicon only results in From-Deepest-Fathoms turning around and talking as if the Dragonborn has done her a favor and beginning the quest.

Bugs

 * The Ancient Knowledge perk is currently bugged; The armor bonus does not only apply to Dwarven Armor, but enhances armor rating from ALL heavy AND light armor types, rather than to only dwarven armor; This commonly leads to the assumption that the effect doesn't work at all, since the way one expects to see the change of the armor bonus is by comparing armor rating before and after equipping different armors – this assumption is incorrect because the armor bonus is still applied, even if it isn't applied correctly. In addition, instead of increasing the rate at which the Smithing skill increases, it increases the Smithing skill by 15 points.
 * This information needs some attention. On my version (first patch, PC) effect is working as intended, at least dwarven part. Increases armor values of all types of dwarven armor by roughly 25%, both equipped and not. Doesn't increase armor values of any other types of armor. Smithing part needs confirmation.
 * Does not increase armor values on other types of armor (PS3)