The ??? quest category is typically used as the first in a series of quests (usually a long series), where the game developers don't want to make it easy for you to find the clues that allow you to progress the quest. The triple question marks make it possible for developers to keep players from changing the quest sorting on their quest journal from the category to the current zone, which would give them a hint as to the zone (or zones) in which they should look for clues.

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