Chapter III: The Foundry Falls

This achievement is obtained by completing the legendary quest line through [zone=6967] raid. The quests are as follows:

• • Reverse Piracy: Collect [item=115494], which drops from the Dread Grotto encounter
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  • Heart of the Fury: Collect [item=115479], which drops from the Blast Furnace encounter
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  • Flamebender's Tome: Collect [item=115493], which drops from Flamebender Ka'graz encounter
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  • Might of the Elemental Lords: Assemble 3 [item=115509] by fusing together 300 (x3) [item=115510] which drops from any of the Foundry bosses and some Garrison 3-followers missions
  
  Note: Completing the quests line rewards the ilvl 710 ring.

I've made a video showing the full version of Khadgar's legendary quest line given during Warlords of Draenor. It contains both sides so Alliance and Horde, hope you'll enjoy!

https://youtu.be/LX208g0W6i4

Gul'dan and his shadow council were released to disconnect the dark portal, but by doing so we created a big problem. Gul'dan was willing to sell his own people to the legion and he will stop at nothing to gain more power. Khadgar recruits heroes of both the Alliance and the Horde to try and stop this evil warlock. His attempt nearly cost him his life to the assassin Garona Halforcen, but thankfully we were able to save him and even capture the half orc. After liberating her mind, we find out that the Iron Horde isn't happy with their current warchief and Gul'dan makes full use of this. It's become even more difficult to stop the evil warlock and his demonic masters...

You have it backwards. Its collect 900 runes to form 3 tablets.