Challenge Conqueror: Gold

The different transmog sets for the classes can be found here:

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• Death Knight: Lich Lord Plate
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• Druid: Cycle Armor
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• Hunter: Howling Beast Set
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• Mage: Elemental Triad Regalia
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• Monk: Regal Lord Armor
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• Paladin: Holy Warrior Plate
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• Priest: Light Regalia
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• Rogue: Silent Assassin Armor
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• Shaman: Windfury Set
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• Warlock: Betrayer Regalia
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• Warrior: Golden King Plate
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Completing all the Gold achievements will in addition to this achievement, also reward you (per dungeon) a teleport spell so you can quickly get to the instance when ever you want. These spells are fly-out spells, meaning they are grouped and only take one actionbar slot -that's nice. The fly-out group consists of:

[spell=131204]

Temple of the Jade Serpent: Gold

[spell=131205]

Stormstout Brewery: Gold

[spell=131206]

Shado-Pan Monastery: Gold

[spell=131222]

Mogu'shan Palace: Gold

[spell=131225]

Gate of the Setting Sun: Gold

[spell=131231]

Scarlet Halls: Gold

[spell=131229]

Scarlet Monastery: Gold

[spell=131232]

Scholomance: Gold

[spell=131228]

Siege of Niuzao Temple: Gold

The complete list (in case this gets outdated) of Gold dungeon challenge achievements can be found here.

Also, if you ever wonder what heroics you have yet to complete and unlock challenge mode for, simply paste this into the chat on your character:
/run for x,i in ipairs({6456,6470,6756,6758,6759,6760,6761,6762,6763})do local _,_,_,c,_,_,_,_,_,_,_,_,m,b=GetAchievementInfo(i)print(x,"\124cff"..(m and"00ff00C"or"ff0000Inc").."ompleted\124r",GetAchievementLink(i))end

Is it only me who find it a bit amusing that you get the teleport reward after completing gold? I mean, once you finally get gold, I bet you'd be kinda fed up with the place and not want set foot there again for a very long time.