“Black Book Filament and Filigree” is a Side quest in Skyrim - Dragonborn
Head into the Darkness to the South. Go to the right-hand side of the stairs to find some loot, including a skill book Three Thieves. There are a few random Soul Gems there as well if you need some. Rush up the stairs to the landing at the top of them.
On the right-hand side, at the top of the stairs, you will find the scrye to open the gate in front of you. Just past the scrye is a table that can have a spell scroll and nearby it is a pod with more loot inside.
Head forward and go through the Darkness to the first pool of light. There are a few moving lights in this area. There are also a few pools of fluid on the path. Be mindful of the tentacles that can emerge from the pools. Crossing this stretch of Darkness can take some time. Be sure to either run or use an alternate source of light if you do not want to deal with the damage.
Going up the starts will lead to a safe zone. There is another pool of fluid with a short set of stairs beyond it.
Heal and wait a moment. A leveled Lurker will emerge from the pool that you need to defeat to move forward.
Go up the second set of stairs. On this top landing you will find the Black Book itself.