The Daedric Quests are quests that are completed at the request of one of the Daedric Princes, in reward for which you will receive 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 players will find out about the Daedric quest.
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 related to daedric quests are:
- Daedric Influence (10 points/Bronze) — Acquire a Daedric Artifact
- Oblivion Walker (30 points/Silver) — Collect 15 Daedric Artifacts
- 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:
- Can be completed without receiving an artifact: A Daedra's Best Friend, Pieces of the Past, The Taste of Death, and Waking Nightmare. See individual quest page for details.
- Never gives an artifact: Darkness Returns (this quest is technically considered 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)
- 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)
- Even if you sell, give away, or otherwise lose an artifact after receiving it, you will still receive the achievement. All that matters is whether you ever had possession of the artifact.
- 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.