How do you start the quest The Thunder Forge?
My server has unlocked all the stages of the Isle of Thunder.
The quest in question is a part of legendary quest chain. To unlock it, you have to do all quest prior to "The Tunder Forge".
To start legendary quest chain you have to be level 90 and start quest "Stranger in a Stramge Land". After it, you simple have to follow all the quests in the chain which will eventually lead to The Tunder Forge.
If you have already done some quests in the chain, check your Quest Log. Quests which are part of legendary chain will be marked as < legendary > in a Quest Log. If you have none of them, visit Wrathion located in the Tavern in the Mists.
If you are sure you done everything what was asked from you, but you still can't access the quest, simply contact Game Master via in built Customer Suport (red question mark on your micro bar).
The quest seems to be part of a chain. Here is the order of the quests.
[5.0] Stranger in a Strange Land
[5.0] Fear Itself
[5.1] The Lion Roars or Glory to the Horde
[5.2] The Thunder Forge
[5.3] Celestial Blessings
From your own link: talk to Wrathion in his whelp form at the Thunder Forge. If you can't find him, do a