You can’t buy Mage tower challenge anymore, but there is still it’s related appearances recolors, so if you have done mage tower then you still can get :

• Defeat Kil’jaeden, the final boss of Tomb of Sargeras on any spec with any artifact weapon related to the class you earned the challenge tint on.
• Join an RBG group and win 10 times on any spec with any artifact weapon related to the class you earned the challenge tint on.
• Clear all Legion dungeons (difficulty does not matter) on any spec with any artifact weapon related to the class you earned the challenge tint on.

Requirements: 

• Have the challenge appearance unlocked.

Boost time depends from choosen options. 

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