Skyrim:Black Book: The Hidden Twilight (quest)

Quick Walkthrough
# Read the Black Book The Hidden Twilight in Tel Mithryn.
# Make your way through Apocrypha‎.
# Learn the Black Book's hidden knowledge.
# Choose your reward.

Detailed Walkthrough
There are eight chapters/zones here, so be prepared for a trek. You'll materialize in a room with a platform and two fonts of magicka. A scrye is past the platform, straight ahead. Activate the scrye and the door behind it will open, revealing a book on a pedestal: the next chapter. Before going in, search behind and below the pedestal to find a pod containing loot. Also note that there is a secret way to skip several chapters from Chapter I, as detailed further on.

Chapter II starts out as a corridor twisting past fonts of magicka built into the walls of books. When the path opens up, beware two seekers wandering the room ahead. Dispose of them and grab the soul gem on an altar, then head east where a ramp leads up. At the end of the ramp is another scrye. The nearby bridge is curled under at first and will uncurl when the scrye is activated; you have to time your jump carefully or you will fall down to the lower level and will have to go around again. Another bridge of the same type is on the other side. Once across, there are ramps down to the left and to the right.  The left is a dead end, but it contains a pod and an altar with two soul gems. If your Conjuration skill is 40 or higher, the altar will also have a copy of the Conjure Seeker spell tome. You will also be able to see a lurker through the wall behind you, but you will not be able to engage him at this time; he's in Chapter III. The right has the book that leads to Chapter III.

After entering Chapter III, head forward around a pillar of books. Before you go through the opening, there's a pod in a shadowed corner to the left. There's another seeker on a platform in the middle of a lake of dark water just ahead. To get the platform's stairs to extend, find the scrye overlooking the area, which is up the short ramps and stairs to either side. You can also snipe at the seeker from here; it will not be able to "teleport" to you. Whether you choose this route or not, go back down. Beware the tentacles which will lash at you as you cross, and be careful not to fall off the side of the platform. There are also three soul gems on the platform.

On the other side (avoid the tentacles!) is a large room with a closed gate in front of you, and a lurker haunting the area to the right. Kill it and head past where it was to find a ramp, at the top of which is a scrye to open the gate.

Before you head through, however, there is an optional area accessible from here which leads to five skill books, corresponding to each school of magic. Stand at the bottom of the stairs to the platform where the ramp is, and look east. You will see a narrow gap between two book pillars, lit by two glowing-tentacle lamps. This narrow path leads you under the platform of the "main" exit route, and to Chapter IV. When you reach Chapter IV, you will be right in the path of a seeker, but you will also find a scrye. It opens a gate in another chapter (visible from Chapter IV but inaccessible now). This small room with seeker and scrye is the entirety of Chapter IV.

Go back to Chapter III and go through the gate you opened before your venture into Chapter IV. The next gate can be opened by a scrye up the ramp to the south-southwest. A seeker is in the maze-like section ahead, so kill it and go east. There is what looks like a dead end up ahead, but as you approach the rest of the corridor will appear. There is a pod on the north side just before the newly-visible section of corridor. At the end are Chapter V and Chapter VI.

thumb|right|Choose your new power.
If you've been to Chapter IV, your extra reward is in Chapter V. It lands you in front of another seeker, as well as a scrye to open the gate. The five skill books are to the left: A Hypothetical Treachery, Racial Phylogeny, Breathing Water, Before the Ages of Man, and 2920 Hearth Fire v9. There's also a loot pod in the same room. When you're done, go back to Chapter III, and then go to Chapter VI. This area is very small, but leads to both Chapter VII and Chapter VIII. Visit the former first and kill the seeker before activating another scrye. It raises a bridge in Chapter VIII, where you'll need to go now.

Another seeker ambushes you here. On the left side before you get to the bridge you can grab two soul gems (one black and one grand) off a table. A lurker will burst from a pool on the left side of the platform as you cross the bridge. It will not cross the little gap in the bridge, so if you have a bow or long-range spells, you can retreat and snipe at it while it is stuck. Depending on your current level, a second lurker may appear from the right pool as well.

Your target is across another bridge straight ahead: a book, The Hidden Twilight. Other loot is scattered around the same area, including a vessel, four soul gems (two black and two grand), and three scrolls (a Scroll of Ebonyflesh, a Scroll of Grand Gealing, and a Scroll of Paralyze). Upon opening the book, three green orbs will appear in front of you, one for each reward. Choose one, then activate the open book to travel back to Tel Mithryn.

If at a later time you want to change your reward, reread the book and you will be transported to this location again. You will not have to trek through the previous chapters.

Reward
Your choice of one of three greater powers:
Mora's Agony — Summons a field of writhing tentacles that poisons foes.
Mora's Grasp — Freezes the target between Oblivion and Tamriel for 30 seconds, making them immune to all damage.
Mora's Boon — Fully restores your health, magicka, and stamina.

Achievements
One achievement is related to this quest:
 

Quest Stages