Iron Horde Invasion

The portal is near the Mage trainer in the Cleft of Shadows (where the Blasted Lands portal previously was), not in the Drag itself.

As of Patch 8.1.5 the [item=111660] is STILL OBTAINABLE!
Follow the steps below to obtain, rather than run around aimlessly for hours like I did.

FIRSTLY - It does NOT matter whether you have completed your Garrison quests, got your Garrison Hearthstone and completed the WoD quest chain. You can still pick this quest up.
I was also lvl 120.

To start - head to the Stranglethorn Vale Zepplin in Orgrimmar. You want to catch this to Stranglethorn Vale now that they have gotten rid of the portals to the Blasted Lands.
Fly to the Dark Portal.
Once there head east to Shattered Landing on the beach. You will then find a small Horde outpost on a hill.
You’ll know you’re in the right place as you’ll see the Zepplin hovering above your head.
Thrall will be there waiting for you (coords 71.8, 47.8), with the Attack of the Iron Horde and Under Siege quests ready. You need to accept both quests.
The third quest Peeking into the Portal starts with Rokhan and is just to Thralls left.
Iron Horde Invasion the original starter quest is no longer available.

Hope this saves everyone some time!
Happy Pet Battling!

The followup quests are no longer available. I did them this weekend on two Alliance characters but my Horde character was only able to turn this in and wasn't able to get the next quest. Too bad, as he still had some 496 Timeless Isle gear that could have benefited from the 515 upgrades.

Edit: The problem was that I had the quest to attack Warlords of Draenor: The Dark Portal, which hides the subsequent quests. Drop the Dark Portal quest and you'll see the quests in Blasted Lands. Or turn in the Dark Portal quest (ie. complete the intro series through the Tanaan Jungle), you'll once again see and be able to complete the Blasted Lands quests.