A Curious Crystal

Please keep in mind that the quest is not repeatable during one DMF festival. After you complete the quest you cannot get it for the second time, as it states when you use an item for the second time "You have already completed this quest". In other words - you can do the quest once every month.
Now, to all the naysayers, Blizzard has confirmed that these quests are to be repeated each month. Source
They have also stated that the quest drops that happen in the dungeons/raids/BG’s happen all the time, even if the faire isn’t in town. Source

What is this? This quest A Curious Crystal is started by the item Imbued Crystal - that you can turn in with the Darkmoon Faire whenever it is in town.

How do I get it? In order to be eligible to loot this, you must have the [item=71634] in your bags inventory - which can be obtained in one of two ways:

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• Receive it as a reward from Gelvas Grimegate after completing The Darkmoon Faire for Alliance or The Darkmoon Faire for Horde.
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• Receive it from Selina Dourman just by talking to her, she stands at the first tent on the left-side of the entrance to the Darkmoon Faire.
  Note: if you have The Darkmoon Faire quest in your quest log, Selina will not offer to give you the guide - you will either need to drop the quest and speak to her again or simply hand in at which point you get the guide anyway.
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Where do I get it? Imbued Crystal seems to drop various level-appropriate 5-man dungeon bosses. Drop locations will continue to be updated as they are confirmed:

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• Taragaman the Hungerer -- Ragefire Chasm (dungeon level 15-16) this boss no longer exists
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• [npc=4832] -- [zone=719] (dungeon level 22-25) this boss no longer exists
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• [npc=3977] -- [zone=6109] (dungeon level 30-33)
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• Razorlash -- Maraudon (dungeon level 36-39)
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• [npc=12201] -- [zone=2100] (dungeon level 36-39)
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• Ras Frostwhisper -- Scholomance (dungeon level 40-43) this boss no longer exists
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• Immol'thar -- Dire Maul West (dungeon level 44-47)
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• Balnazzar -- Stratholme Live (dungeon level 48-51)
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• High Interrogator Gerstahn -- Blackrock Depths (dungeon level 53-56)
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• [npc=17377] -- [zone=3713] (dungeon level 58-70)
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• [npc=18344] -- [zone=3792] (dungeon level 61-70)
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• Warp Splinter -- The Botanica (dungeon level 67-75)
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• Ormorok the Tree-Shaper -- The Nexus (dungeon level 71-73)
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• Kael'thas Sunstrider -- Magister's Terrace (dungeon level 72)
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• Loken -- Halls of Lightning (dungeon level 79)
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• Loken -- Halls of Lightning (dungeon level 80 heroic)
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• Scourgelord Tyrannus -- Pit of Saron (dungeon level 80 heroic)
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• Asaad -- Vortex Pinnacle (dungeon level 82-84 and 85 heroic)
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• [npc=39732] -- [zone=4945] (dungeon level 83-84 and 85 heroic)
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• Queen Azshara -- Well of Eternity (dungeon level 85 heroic)
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• [npc=56439] -- [zone=5956] (dungeon level 85-87 and 90 heroic)
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• [npc=3977] -- [zone=6109] (dungeon level 90 heroic)
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Why do I want it? There are several reasons that you may want this item:

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• Receive 10 Darkmoon Prize Tickets upon handing it in - these tickets allow you to purchase mounts, pets and other collectibles whenever the Darkmoon Faire is on.
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• Receive 250 reputation (or more depending on your racial/guild bonuses etc) with the Darkmoon Faire - great for anyone who is either a rep junkie or is working towards The Insane title awarded by the Insane in the Membrane achievement.
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• Having completed the quest that this item starts will count towards the achievement Darkmoon Despoiler.
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• Gold gold GOLD - since this item isn't BoP (bind on pickup), it is tradable and sellable via the auction house. If you don't care about the above three points, then you can profiteer from someone who does!
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What else do I need to know?

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• High level characters can no longer are not supposed to be able to farm lower level dungeons for this one since a recent hotfix. This should now only obtainable in level-appropriate dungeons (though there continue to be reports of 85s picking up Imbued Crystal in Scarlet Monastery in particular), and as always, success is at the mercy of RNG.
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• The quest can only be turned in once per Darkmoon Faire. You can certainly collect more of these, however if you've turned in for the current faire, you'll have to wait till it's next in town.
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• This item can be looted anytime - that is to say, the Darkmoon Faire does not have to be currently on in order for you to be able to find these puppies.
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What other Darkmoon Faire drops are there?

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• Fallen Adventurer's Journal -> The Captured Journal = 5 x Darkmoon Prize Tickets
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• Banner of the Fallen -> A Captured Banner = 5 x Darkmoon Prize Tickets
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• Captured Insignia -> The Enemy's Insignia = 5 x Darkmoon Prize Tickets
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• Monstrous Egg -> An Exotic Egg = 10 x Darkmoon Prize Tickets
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• Mysterious Grimoire -> An Intriguing Grimoire = 10 x Darkmoon Prize Tickets
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• Ornate Weapon -> A Wondrous Weapon = 10 x Darkmoon Prize Tickets
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• Soothsayer's Runes -> Tools of Divination = 10 x Darkmoon Prize Tickets
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• A Treatise on Strategy -> The Master Strategist = 15 x Darkmoon Prize Tickets
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Will frequently amend and extend this post as users request it or as I learn more.

I was auctualy looking for this, so it shoudlebt be downvoated