Skyrim:Daedric Quests

The Daedric Quests are quests that are completed at the request of one of the Daedric Princes, for some kind of reward, usually a Daedric artifact.  Sixteen Daedric Artifacts are available in total (note that the Skeleton Key is not considered to be a Daedric Artifact in Skyrim). Daedric Quests are not considered Side Quests.

Most of the Daedric quests also have an associated "Pointer Quest", which is a miscellaneous quest objective you may be given that leads you to the actual Daedric quest.  It is not always necessary to obtain the Pointer Quest before starting the main quest, but it is the most likely way that you will find out about the Daedric quest. In some cases, you can start the actual Daedric quest by simply visiting the shrine or other location where a Daedric worshipper can be found.

Most of the quests can only be removed from your 'to do' journal upon completion. However, a few of the quests can be failed. This is usually achieved by killing the mortal quest giver. For example, upon entering Silus Vesuius' home, you can kill him, resulting in the failure of Pieces of the Past.


 
 

Achievements
Achievements related to daedric quests are:
 
 
It is possible to miss this achievement. To receive it, you must not simply complete 15 Daedric Quests, but must also choose the quest ending that gives you the Daedric Artifact.
Quests which do not simply result in you receiving one artifact are:
Those which can be completed without receiving an artifact: A Daedra's Best Friend, Pieces of the Past, and Waking Nightmare.  See individual quest page for details.
Those which can be failed: The Taste of Death, Pieces of the Past, Discerning the Transmundane, and The House of Horrors.
Those where an NPC can die before giving you the quest: A Daedra's Best Friend and The Whispering Door.
The one which never gives an artifact: Blindsighted (the game considers this quest to be a Thieves Guild quest rather than a Daedric quest, so the Skeleton Key does not count as one of the Daedric artifacts for this achievement).
The one which can give two artifacts: Ill Met By Moonlight (both Savior's Hide and Ring of Hircine count as artifacts, and both can be obtained at the same time).
You do not need to "possess" the artifacts, just choose the ending that results in the quest adding to your artifacts total. However, all but one of the quests have the incremental scripted to taking possession of an artifact. The odd-one-out is The Cursed Tribe, where the quest ends but the artifact is placed in front of you (rather than in your inventory). You do not need to keep the artifacts once you have them, though they can be used to verify which quests have been completed "correctly". 
The game tracks the number of Daedric Artifacts and the number of completed Daedric Quests separately.  The only statistic visible to the player in-game is the number of "Daedric Quests Completed" (under General Stats).  Since the quest statistic is incremented even if you complete a quest without receiving the artifact, it may not reliably indicate your progress towards this achievement.
Auriel's Bow  also counts towards this achievement, despite the fact that it is an Aedric, not Daedric, artifact, and is therefore tied to the plane of Aetherius, rather than the planes of Oblivion.