this place is not quite as easy to find as everyone says... i ran around quite a bit after reading every one of these posts...

There was a post above that was on the right track but got a little hazy.

Second cave To the left (theres 5 caves in the cauldron, get the 2nd one.

Best way is to take the elevator located next to one of the 3-4 cranes on the north edge.

Once inside the cave, you might notice along the back wall "Quarry Gate" go through that. Follow the quarry around, it bends to the right mostly.
High levels can sneak around easily.

You'll see a lot of "Slave Worker"s and "Dark Iron Slaver"s. Eventually you come to a big room with "Dark Iron Taskmaster"s and an elemental named "Obsidion" on its back getting repaired by "Enslaved Archeologist"s. (they walk back and forth from "Ob" to some tools thing or something) you'll see to the south-east of that guy, a big ramp. Go up that.

At the top is a small room with some other guy "Blackheart" i think was in his name, i killed him too fast to care. (i was really frustrated)

You venture into what seems like a bunk room. (beds all along the walls benches/stools in the middle) No worries you're on the right track. Slightly past there, follow the right wall and you'll see a balcony, maybe a mob or two. Pass that, keep following the right wall, and you'll see another balcony with Overseer Maltorius, the plans are behind him.

In cosmos, 42,35 was the loc where i could stare right at him.

I hope this helps.
- --Zindaklu of Thunderhorn

Awesome joke in the quest name. Not sure how 20 comments went by without any mention of this.

They are right behind Overseer Maltorius.
Loc: 40,36

It's down in The Slag Pit, on a table behind three Elite mobs.

http://imagesocket.com/view/whattheflux1110.jpg 
http://imagesocket.com/view/whattheflux2740.jpg

If you just want to dash in and get the scroll, then take off, you might have some ways of dealing with the elites, depending on your class.

Priests can use psychic scream

Warriors can use intimidating shout

Mages, depending on how good your timing is, might be able to get away with using frost nova

Hunters and Warlocks may be able to get away with distracting the mobs with their pet long enough to grab the item and make a getaway

Completing this quest will allow you to turn in various items in return for reputation with the Thorium Brotherhood, that when high enough allows you to buy various crafting recipes.

The first turn-in quests:

Restoring Fiery Flux Supplies via Heavy Leather (https://classicdb.ch/?quest=8242)
Restoring Fiery Flux Supplies via Iron (https://classicdb.ch/?quest=8241)
Restoring Fiery Flux Supplies via Kingsblood (https://classicdb.ch/?quest=7736)

The turn-in quests are level 60, and if you have problems with this quest, you can just wait until you're in your high 50's and grab the plans without aggroing the the nearby mobs, as they will be gray to you.

On the other hand, the nearby elite, Maltorius, is also a target of the quest WANTED: Overseer Maltorius (https://classicdb.ch/?quest=7701), so you might want to kill him anyway.

The item is behind the big nasty Overmaster Maltorius (50elite) and his two lovely guards (48elite), in the Cauldron, Slag Pit, 40,35.

for everyone that doesnt give a schit about how someone did this, its at 41,36 in the slag pit.

Enter the cave for the Secret Plans: Fiery Flux @ 49,46. Stay to the left and don't fall D. Becarful of the elites, on the balcony part.. If your in a group might as well do https://classicdb.ch/?quest=7701 as well