Darkness Returns

Objectives

 * Enter the Twilight Sepulcher
 * Speak to the Nightingale Sentinel
 * Follow the Pilgrim's Path
 * (Optional) Retrieve Nystrom's Journal
 * Return the Skeleton Key to Ebonmere
 * Listen to Nocturnal
 * Speak to Karliah
 * Choose Nightingale Role

Walkthrough
To complete this quest, the Dragonborn must take the Pilgrim's Path to the Twilight Sepulcher in order to put the Skeleton Key back where it belongs. As the Sepulcher is entered, the ghost of Gallus Desidenius is encountered, the Nightingale murdered by Mercer Frey. He warns the Dragonborn that, because the Key is missing, the other ghosts have been corrupted and are now hostile. When asked about the obstacles to reaching the Ebonmere Lock, he advises to look for a journal that belonged to another adventurer, Nystrom. His skeleton is to the left of Gallus.

Nystrom's journal holds some clues to the trials:

"Shadows of their former selves, sentinels of the dark. They wander ever more and deal swift death to defilers."

"Above all they stand, vigilance everlasting. Beholden the murk yet contentious of the glow."

"Offer what She desires most, but reject the material. For her greatest want is that which cannot be seen, felt or carried."

"Direct and yet indirect. The path to salvation a route of cunning with fortune betraying the foolish."

"The journey is complete, the Empress's embrace awaits the fallen. Hesitate not if you wish to gift her your eternal devotion."

Nystrom also says that the man who gave him the clues mumbled nonsensical phrases like "night is the new day" and how he was the "ghost of the sun"."

First Trial - Sentinels
The first trial is facing the shadows of their former selves, sentinels of the dark - Nightingale ghosts, hostile and powerful. Either sneak past or kill them. If they are killed, there is valuable loot on both sides of this area.

When entering the first trials room, it is relatively easy to sneak attack the Nightingale sentinel with a bow as they are directly in front of the Dragonborn when they enter. Then they can continue on to the next trial or, in the chance of receiving more loot, kill the other sentinels.

Look behind the desk in the room to the left. There is a button that lowers a stone wall. There is loot and an alchemy table in the small room there.

Second Trial - Light versus Dark
"Above all they stand, vigilance everlasting. Beholden the murk yet contentious of the glow."

For the second trial, night is the new day: avoid the light and stay on the darkest path. One can enter sneak and go through the mist. Standing will cause large amounts of damage. Watch for trip wires when going up the staircases. During this trial there are no enemies. Candlelight and Magelight spells can also help if having trouble finding the way around, as their own lights aren't harmful in this area.

Third Trial - Hidden Door
For the third trial, in the room with the statue and a dead bandit lying in front of it, there are two large sconces to either side of the statue. To the left of the left hand sconce there is a chain, and at the right hand side of the right sconce there is a second chain. Pull the two chains to put out the lights of the sconces, so giving her (Nocturnal) what she desires most (darkness). The door behind the statue will then open.

Fourth Trial - Sentinels vs. Traps
For the fourth trial, there are some options:


 * Straight ahead through the traps: step carefully and use Whirlwind Sprint or Become Ethereal. Be careful with the doors. The swinging axes can transmit diseases, even to a werewolf or vampire.


 * Before the first trap, there is a Master locked iron door to the left that can be used to avoid the traps (open it with the skeleton key). Sneak past the two sentinels.


 * As above, but kill the sentinels and loot the pit. Among other things it contains two skill books: Proper Lock Design and Sacred Witness).

Fifth Trial - Inner Sanctum
After completion, the entire area will be sealed. To get the loot, all of the sentinels (except for Gallus) must be killed and the area searched.

Enter the Twilight Sepulcher Inner Sanctum, fall into the pit and search Anders' skeleton; read the message. Wait a few seconds until the Skeleton Key reacts. The ground will disappear and the Skeleton Key will return to the Ebonmere Lock. That will summon Nocturnal, who will bid the Dragonborn to drink from the Ebonmere, "for this is where the Agent of Nocturnal is born".

Then speak to Karliah, and she explains the choice that must be made between the Agent of Strife, Subterfuge and Stealth. To pick one, step on the corresponding moon. They give greater Powers, usable once a day only. The Dragonborn can choose to change the chosen power after a day so there is little pressure in choosing the best ability. Next, talk to Karliah. After Karliah is spoken to, Gallus' ghost will appear and they will speak shortly before he goes to Evergloam. Karliah, then, goes back to Nightingale Hall. The Dragonborn can then leave through a portal.

Powers
Nightingale of Shadows
 * The crescent moon represents this power, which makes the Dragonborn the master of remaining unseen. Once a day they can become invisible while crouching for 120 seconds, much like The Shadow Stone.

Nightingale Subterfuge
 * The half moon represents this power, which allows the use of shadow to cloud the judgment of those around them. Once a day, it can be activated for 30 seconds in order to make people and creatures in the spell's area of effect attack anyone nearby.

Nightingale Strife
 * The full moon represents this power, which, when used, instantly absorbs 100 Health points from the target. This power can also be used once a day. Will go through the target and hit others.

Trivia

 * The Pilgrim's Path contains plentiful loot, including skill books, potions, and soul gems. The Pilgrim's Path becomes inaccessible once the key is used to open Ebonmere.
 * It is recommended to not return the Skeleton Key until either reaching the Unbreakable lockpick perk, or are able to become the Thieves Guild Guildmaster, as the Skeleton Key must be returned to do that.
 * Dungeon Lock Outs cannot be entered with this key, as it still counts as a lockpick.
 * During the first test there is a secret door in the room with the Nightingale sitting down. It can be opened by pressing the button on the floor to the left of the seat under the table.
 * After the completion of this quest, a shrine to Nocturnal will appear in the Thieves Guild Cistern. It gives the Blessing of Nocturnal, which makes the Dragonborn 10% harder to detect for seven hours.
 * In the second trial, only the light from the sconces can damage you, so you can still use torches, spells, etc. (Only tested with candlelight spell)

Bugs

 * After Nocturnal is done speaking, the quest marker will follow her into the Ebonmere instead of moving to Karliah, so that she can't be spoken to. Re-loading from before returning the key to the Ebonmere can fix this.
 * This quest may be removed from the quest menu. Enter the Twilight Sepulcher to continue the quest.
 * When using the portal to the entrance, the Dragonborn may fall through the floor of the game to nothingness and die on the spot.

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