The Heart Forge

For Alliance, you need to finish Uniting Kul Tiras.

Afterwards, hit 120 & start the The Wolf's Offensive from Genn Greymane in Boralus,
which will start the introduction quest to Nazjatar and awarding you this achievement at the end of the quest line, which lead to another quest line to get [item=169223].

In order to get this quest in Patch 10.2.6 I had to do the following:

Progress the Najatar questline untill Magni turned up.
Progress the questline with Magni untill I fully unlocked the Heart Forge (gaining the Heart Forge achievement.

I was then able to return to Cyrus' office where the quest was available from a little letter lying on the table next to the Scouting Map.

Yep, skip all the old stuff and jump right to going to Silithus to meet Magni Bronzebeard so to start the Legendary Cloak questline!

On Alliance, go to the NPC 'Earthen Guardian' at 75, 15 in Boralus Harbor (standing outside, just east of the Snug Harbor Inn in the Tradewinds Market).

On Horde go to the NPC 'Earthen Guardian' at 51, 89 in the southern end of Dazar'alor -- just SW of the kitchen in the Grand Bazaar.