First, head over to the Riften Fishery area outside of Riften (to the south west) where you will find an Argonian walking the docks named From-Deepest- Fathoms . Talk to her. She will immediately ask you to "return the lexicon" and just seems overall… off. Agree to take the lexicon back to Avanchnzel for her and you’ll start this quest.
Avanchnzel is a dwarven ruin to the west of Riften. Make your way over to it or fast travel if you’ve already discovered it and enter. Head through the tunnels to the first opening where you’ll see red spirits talk amongst each other, From-Deepest-Fathoms included. It seems that she and three others were hired to steal this lexicon.
Listen to the spirits until the leave, then go down the path and kill two dwemer spiders. Continue onto a bridge past some doors and you’ll see two of the spirits talking and heading straight, saying this place was like a library on the dwemer. Here you can follow the spirits or go left. Left leads up to the Avanchnzel Balcony and has some more spiders for you to kill, so go straight. You’ll be in a hallway full of steam.
Follow it and kill the two dwemer guardians who patrol the hallways until you get to some hallways leading down. There are more spiders down here as well as a door on the way down you can lockpick to access a chest.
Head forwards and soon you’ll be overlooking a wide cave area. Use the ledge here to get in some ranged shots on the four spiders hanging out down here. After killing them, go through the door to see another scene from the raiding party. Afterwards, check the left alcove for a chest and go through the door beyond to enter the Avanchnzel Animoncultory.
Head down and kill the spider and sphere guardian you find, then pick the door on the right. After you pick the lock, the spirits will complain that none of them can pick a lock (hilarious!). Head onwards and keep your eyes open for a chest up some stairs to the right and soon you’ll be at a camp where you’ll see another scene between the spirits.
Head on and enter a room on the right (enter it quickly as a trap is sprung) to find a chest and, on the table, a RESTORATION SKILLBOOK named "Racial Phylogeny" . Keep going through a storage area with a skeleton in it. There’s also an alchemy lab to the left if you need it.
Head further down a ramp and through a door to face two more spiders and a sphere guardian. Take them out and unlock a door on the right for a chest and some goodies on a table. There’s another locked door on the left too for another chest and more goodies.
Keep going on to see another scene between two spirits and afterwards go through the next door to enter a HUGE wide-open area. Here, you will be above an assembly area of sorts with FIVE dwemer spiders walking around below. Ranged players should sneak to the left and get on the metal bookcase inside the room (jump over it) to get to a nice ranged spot to attack from.
Once you kill all the spiders, be SURE to loot the two dwarven centurion enemies in the middle of this area for some GOOD loot, then go up the stairs to the northwest. Down this hallway you’ll find the corpse of Drennen, one of the looters. Continue on (loot the chest under the shelves on the right) to see a scene where Drennen runs off. Now go through the doors to the Avanchnzel Boilery.
You’ll see a scene in here, then continue on and downwards and kill another sphere guardian. Past that is another storage area (with LOTS of dwemer ingots if you want them) and another scene by the door. Past the door on the floor are a LOT of floor plates that spring a spinning blade trap, complete with pistons that force you into it.
However; it’s a ways down the ramp, and I do NOT see how anyone could fall for this trap. Wait, Watches-The-Roots fell for it apparently. Further on is a wide room, but don’t rush in. There is a Dwarven Centurion alive and well on the left-side of the room, so sneak attack it if you can.
Once you kill it you’ll see Breya’s corpse in here with another dead Centurion. Loot everyone and put the lexicon back into the lexicon receptacle. You will now have completed this quest, but you will also gain a prize: knowledge!
Knowledge gained from the Lexicon gives you a 25% bonus when wearing Dwarven Armor and Blacksmithing increases 25% faster.
A VERY nice reward! Past the lexicon receptacle is a lift that takes you back to Avanchnzel (a shortcut back to the path split from WAY earlier) which you can use to take you back to Skyrim.