Darkness Returns

Overview

 * Prerequisite: N/A
 * Quest Giver: Karliah
 * Reward: Nightingale Strife / Nightingale Shadow / Nightingale Subterfuge
 * Reputation Gain: N/A

Walkthough
You must take the Pilgrim's Path to the Twilight Sepulcher in order to put the Skeleton Key back where it belongs. As you enter the Sepulcher, you stumble into the ghost of Gallus, the Nightingale murdered by Mercer Frey. He warns you that, because the Key is missing, the other ghosts have been corrupted and are now hostile to you. When asked about the trials ahead of you in order to reach the Ebonmere Lock, he advises you to look for a journal that belonged to Nystrom. His skeleton is to your left as you look at Gallus.

Nystrom had 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"."

For the first trial, you face the shadows of their former selves, sentinels of the dark - Nightingale ghosts, hostile and powerful. You can either sneak past or kill them.

For the second trial, night is the new day: avoid the light and stay in the darkest path. Sprinting and using the Whirlwind shout will make it easier.

For the third  trial, In the room with the statue and a dead bandit lying in front of it there are two torches to either side of the statue. To the right of the right torch is a chain and to the left of the left torch is a chain, pull the two chains to open the door behind the statue.

For the fourth trial, avoid the traps: watch your step and use Whirlwind Sprint. Be careful with the doors. Before you find the first trap, there is a locked iron door to your left that can be used to avoid the traps, but you may be seen by ghosts.

After that, fall into the pit and search Anders' skeleton, loot the message. Wait a few seconds until the Skeleton Key reacts. The ground will disappear and you will return the Skeleton Key to the Ebonmere Lock. You will summon Nocturnal, who will bid you to drink from the Ebonmere, "for this is where the Agent of Nocturnal is born".

You must then speak to Karliah, and she explains to you the choice you must make between the Agent of Strife, Subterfurge and Stealth. To pick one, you must simply step on the corresponding moon. They give you greater Powers, usable once a day.

The crescent moon represents the Agent of Shadow/Stealth, who's the master of remaining unseen. Once a day, you can activate it for 120 seconds and you'll automatically become invisible while stealthing.

The half moon represents the Agent of Subterfuge, who utilizes shadow to cloud the judgement around him. Once a day, you can activate it for 30 seconds in order to make people and creatures on the spell's area of effect attack anyone nearby.

The full moon represents the Agent of Strife, and once a day you can use it, instantly absorbing 100 Health points from the target.

After you speak to Karliah, Gallus' ghost will appear and they will speak shortly before he goes to the Evergloam. Karliah, then, goes back to Nightingale Hall.

Bugs

 * Near the end of the quest after Nocturnal bids the player farewell and returns back into the ground (or well) the quest marker will get stuck in the ground and Karliah doesn't came to speak with the player. This leaves the player stuck in this room for eternity and even after many reloaded saves the same result occurs.