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Operation: Mechagon – The Mechoriginator. If you are interested, here is the complete list of the story requirements to earn The Mechagonian Threat: You can defeat the bosses in the dungeon on Heroic or Mythic difficulty. Once you unlock Mechagon, you need to complete the 16 part storyline, which culminates in defeating the final boss of the Operation: Mechagon dungeon. In addition to making the unlock of Mechagon island easier, you will need a level 50+ alliance character to complete the second part of the Mechagnome unlock anyway. If you didn’t play in BFA and don’t have the achievement yet, I would highly recommend going through this process on an alliance character - even if you have to level one. You will need to complete a significant portion of the story and the Nazjatar intro to open Mechagon. If you play as Horde, this unlock process is much slower. If you have never been there before and play an alliance character, you can visit Gila Crosswires in the Stormwind Embassy to receive a quest called Fame Waits for Gnome One. To complete the storyline, you need to travel to the island of Mechagon.
If you have already earned The Mechagonian Threat on a Horde character, you are good. This can be done on a Horde or Alliance character. The first thing you need to do to earn Mechagnomes is to complete the Mechagon storyline.