Grand Theft Mammoth

• Increase your [faction=2518] & [faction=2517] reputation (by completing activities at the Obsidian Citadel) to achieve The Obsidian Bloodline AND Loyalty to the Prince.
  
  
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• Each of their Quartermasters/Assistant Quartermasters will then offer you one of the 2x items needed to purchase the [item=201837] from [npc=191135].
  
  1. [npc=188623] or [npc=199036] for [item=201839] (800 Gold)
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  3. [npc=188625] or [npc=199020] for [item=201840] (800 Gold)
  
  
  
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• When you use the Harness, you'll get this error message "You must be riding a stolen Tame Magmammoth.". So I tried with all 4x Tame Magmammoths tied to Petty Theft Mammoth, but it was only working on the one in the Burning Ascent area (here's a list of all 4x just in case this might work on another one for you).
  
  1. [npc=198150] /way #2022 33.42 72.07 Burning Ascent
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  3. [npc=198153] /way #2022 23.66 71.44 Dragonbane Keep
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  5. [npc=198155] /way #2022 66.34 24.46 Scalecracker Keep
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  7. [npc=194625] /way #2022 37.10 44.53 Smoldering Perch
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  9. [npc=194623] is the version of the creature when you ride it
  
  
  
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• Successfully harnessing the Tame Magmammoth will permanently add the [item=192601] to your collection, and you will achieve Grand Theft Mammoth. (the Quartermasters will no longer show the items mentioned).

For the Grand Theft Mammoth achievement you need to tame a [npc=198150] in The Waking Shores to get a Loyal Magmammoth as a mount.

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   Reputation farming in the Obsidian Citadel : (Daily/Weekly Quests).
   
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     Reach the maximum friendship with [faction=2517] in the Obsidian Citadel for the Loyalty to the Prince achievement.
     
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     Reach the maximum friendship with [faction=2518] in the Obsidian Citadel for the The Obsidian Bloodline achievement.
     
     
     
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   Shopping items:
   
   
   
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     Buy at [npc=188625] or [npc=199020] for 800 gold the [item=201840].
     /way 25.18 55.84 Upper level Lorena Belle
     /way 26.53 62.45 Lower level Atticus Belle
     
     
     
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     Buy at [npc=188623] or [npc=199036] for 800 gold the [item=201839].
     /way 24.72 56.78 Upper level Samia Inkling
     /way 27.73 56.24 Lower level Xaldrass
     
     
     
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     Exchange these items at [npc=191135] for a [item=201837].
     /way 26.37 55.45 Upper level Yries Lightfingers
     
     
     
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   Catch mammoth:
   
   
   
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     Fly to this [npc=198150] and kill the [npc=186109] standing next to it.
     /way 33.43 72.07 Burning Ascent
     
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       There are three other Magmammoths that count towards the achievement Petty Theft Mammoth.
       
       
       
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     Click on [npc=198150] and use the [item=201837] and you will get the achievement Grand Theft Mammoth and automatically the mount [item=192601] in your mount collection: Loyal Magmammoth
     
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       If you are unable to click on the mammoth, it usually helps to reload the UI with /reload.
       
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In this video here I show you where you can get the items and where exactly you can catch the mammoth: https://youtu.be/uS2RKuQsFh4

I followed the notes on Petty Theft Mammoth#comments that made a point that you often can't ride them due to a bug but you can /rl ( /reload ) and it will work. But ALSO, it says you have to be riding a STOLEN mammoth. What that means is that you can't just go ride it... you have to kill the owner of the mammoth near it first. Then when you ride it, and while you are riding it, you can then use the [item=201837] item you purchased from [npc=191135] to get the mammoth mount. Thanks to the post by Lazey here for the coordinates of the tame mammoths!