No, there isn't a doomsday weapon hidden in it. The whole thing is the weapon.
When people remember Uldum, they usually recall Harrison Ford Harrison Jones and the zone's many Raiders of the Lost Ark references, including the Nazis goblins with inexplicable German accents. All of those Indiana Jones jokes do a lot to hide the fact that the entirety of the zone is built on top of a massive doomsday device.
Now, why is there a world-ending machine buried under the desert? Like a lot of things in the Warcraft universe, the Titans did it. If you're unfamiliar with Titans, they're essentially gods who go around fixing planets according to their whim. Unfortunately, Azeroth is a bit of a problem child when it comes to this sort of thing, thanks to the Old Gods trying to infect the planet with evil like big, tentacled tumors.
The solution? If the Old Gods get too uppity, wipe the slate clean by destroying the world. Needless to say, this becomes an issue because the players and all the other races of Azeroth are not a part of this clean slate. As it happens, most people enjoy not being apocalypsed.
"How can you start fresh if the world's destroyed?" you might ask. The Titans thought of that, and that's why the dungeon in that zone is called "the Halls of Origination", not "the Halls of Blow Everything To Bits". After annihilating everything and jamming all the continents back together, the complex would reintroduce life to all of Azeroth. A similar plan on Earth would involve killing all humans, reconstituting Pangea and bringing back the dinosaurs. Which doesn't sound so bad, when you put it like that.
"But how would the weapon do that?" you ask aloud, unaware that the article you're reading cannot respond. The Titans created a blueprint based on what they determined to be the primordial, natural life of Azeroth, more or less a USB stick containing a save state from before any sentient races arose and started mucking up the planet. That blueprint was the Emerald Dream, the realm of natural magic that all druids draw their power from.
Thankfully, the whole thing was shut down by the players (after accidentally starting it up). Good thing the Titans never needed to activate it, right? After all, mortals seem to have a pretty good handle on this Old God nuisance. We killed C'thun in World of Warcraft and we killed Yogg-Saron in Wrath of the Lich King. Except we didn't.
Sounds like an oxymoron, doesn't it? This isn't a case of them just coming back from the dead like so many comic book heroes and villains -- it's that we didn't actually kill them to begin with.
The Old Gods, as mentioned before, are basically big, evil planet-tumors, corrupting anything they can get their tentacles on. They're heavily inspired by H.P. Lovecraft's Elder Gods; unfathomable evils that can drive mortals completely and utterly insane just from a glimpse of their true forms. Imagine seeing your grandma out of the shower, only it's Cthulhu and you're not sure where her saggy genitals end and the mass of demon tentacles begins.
As mortals are, we can't kill them. According to in-game encounters with servants of these Old Gods: "They do not live, they do not die. They exist outside the cycle." We haven't been able to kill them because death just isn't a thing they do. It'd be like saying, "Today, I slew the color purple. Purple is no more. We offer our apologies to Barney and Grimace."
Only one Old God has ever been destroyed, and even that is iffy at best. It was Y'shaarj (gesundheit), who was killed by the Titans in Pandaria. Even the Titans, essentially gods, couldn't manage to kill one of the Old Gods permanently, and instead had to lock up his still-beating heart. Even without a body, Y'shaarj managed to corrupt Pandaria and coerce the current leader of the Horde to go rampaging across the continent. He was a bit of a dick to start with, but Old God corruption didn't help.
The heroes of Warcraft have faced down countless evils -- the Lich King and endless armies of the undead, a world-shattering dragon and doomsday cultists -- but none of them compare to the unspeakable, unknowable evil of the Old Gods. Except, perhaps, the dire threat of being disconnected in the middle of a boss fight.