The Portal's Power
Enter the Soul Engine beneath the Dark Portal, then shut the Dark Portal down.
Description
This portal is fueled by powers neither mechanical nor arcane.
Look to either side of us, <name>. The screaming souls of slain prisoners are being channeled into the heart of the portal itself.
And they are channeling these souls through the body of Gul'dan.
Find a way into the chamber beneath us, and do whatever it takes to get Gul'dan to stop his channeling. Our last hope rests on you, <name>.
Rewards
You will be able to choose one appropriate item for your class from the following rewards:
| Iron Horde Bolt-Thrower | Iron Horde Tomahawk | ||
| Iron Horde Warcudgel | Iron Horde Hatchet | ||
| Iron Horde Ritual Staff | Iron Horde Waraxe | ||
| Iron Horde Bramblestaff | Iron Horde Carving Knife |
You will also receive:

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Posted Jul 8, 2014 (Patch 6.0.1)Alright, pay attention, because it is surprisingly difficult to find the Soul Engine.
The entrances to the Soul Engine room are almost directly below and to the side of where Khadgar and Co. are standing. Stand where Khadgar is, then choose the left or right descending stairs. If you chose left, keep a close eye to your right side; if you chose right, keep a close eye on your left side. It is very easy to miss the entrance because of the dimness.
Once you enter the Soul Engine, a brief cutscene plays. I don't believe it is complete at this time, but here's a recording I made of it.
After it finishes, Gul'dan will instruct you on how to free him. The three devices are on the floor to the left, right, and back of Gul'dan. You'll probably have to kill one or two of the floating spirits, but they aren't very tough and don't hit particularly hard even on cloth armor.
After destroying the devices, run back up to Gul'dan's pedestal to destroy the Stasis Core. Gul'dan will perform an ability (not an actual cast, just a scripted event) that wipes out the few spirits in the room, do more emoting, and leave via a Burning Legion-style portal... but not before calling you by name. Creepy and disturbing.