An extremly
common videogame concept*. A fetch quest is simply that: A
quest to
fetch something. Go to
foo and retreive
bar. It matters not what bar is, or where foo is located. When someone gets something in return for something else, that is a fetch quest.
However, the phase is commonly used when the Something-to-be-retrieved is some sort of powerful and/or magical artifact**, and the place to retrieve it from is either a deathtrap itself, or the journey to it is arduous and trying.
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* Although fetch quests are by no means limited only to videogames.
** Runner-up for the most commonly fetched item? A princess or other sort of hostage. Only you can Rescue the Princess.