Daedric Artifacts
A Daedric Artifact is a unique item with a very powerful magical ability within the Elder Scrolls series. Artifacts only show up a maximum of one time per game, but are sometimes referred to in other games of the series. In the fifth game in the series, Skyrim, there are 16 artifacts.
An artifact is received after completing a quest for a Daedric Prince. These quests can be obtained from either a pointer quest or from going to the shrine of the prince. You cannot fail most of the quests. However, a few allow for the quest to be failed, usually as a result of killing the mortal quest giver.
There are two acheivements that can be gained from obtaining Daedric artifacts. The first is Daedric Influence, which is obtaining one artifact. The second is Oblivion Walker, which is obtained from obtaining 15 out of the 16 artifacts.