The only spell that even comes close to being able to destroy an entire city with just one casting is "Gate".

And even then... Waterdeep has probably dealt with even that exact problem more than once. I mean, you're probably not the first person who ever thought about destroying that particular city.  

Good luck.

Hmm, if we really want to get out there with ideas, Gate in an M110 Self-Propelled Howitzer loaded with W33 shells.

The M110 had an effective range of greater than 12 miles.  And the W33 is a tactical nuclear artillery shell.  I'm not sure exactly how many it could carry, but I figure that whatever the number is it's sufficient to make the city radioactive enough that nobody can live there.

"I wish... the Trolls had won."

Quote from Kotath >>

Quote from 6thLyranGuard >>

The M110 had an effective range of greater than 12 miles.  And the W33 is a tactical nuclear artillery shell.  I'm not sure exactly how many it could carry, but I figure that whatever the number is it's sufficient to make the city radioactive enough that nobody can live there.

 

I am aware of what the shell is, no worries :) Hence why I suggested a single shot through the gate would likely be enough :)

If you gate it into a remote secure location, yes. The city does have airborne patrols though and even a feather fall spell might be enough to stop the shell from impacting hard enough to go off... If the gate is inside the city so there is no real projectile flight time, it would likely be less risky. Oh and there is also the possibility that a nuclear shell simply does not work, because Gods, because magical world, etc...  Also a similar possibility that the howitzer would not work. And if both did work.... someone gets the blueprints and you have done more than just destroy Waterdeep lol

 Feather Fall can't be used to stop rocks from trebuchets from impacting.  I seriously doubt that any airborne patrols will be able to deal with a heavier, faster projectile.  I think the real danger is if Gond's prohibition against gunpowder works on the propellant used to fire the shell (certainly it shouldn't work on the nuke itself).

Hey kid, want some plutonium?????

That means that gunpowder used to work...

Orcus is probably the best demon lord to summon.

Why was it banned?

Mainly to keep the big change of moving to gunpowder weapons out of the forgotten realms.  That would be a massive change in how warfare would work.  The official reason I am remembering from a Dragon article was that you did not get the high-compression effects from gunpowder.  They left smoke powder for fireworks - and allow an opening for munchkins to waste time and resources - that looks like it would work, but they never get enough energy from it.

I took it as magic (the gods) messed with Boyle's law and you cannot get the energy from the chemical reaction of gunpowder.

Could you theoretically make a firearm using Uranium or another fissile material?

It could be possible...

On Greyhawk they had high-tech weapons in the Expedition to the Barrier Peaks adventure so some advanced stuff works.  Never had anyone try to use gunpowder there though.  It was always magic and swords for my players.

Magnetic linear accelerators would probably be about the only way they could do it, but gunpowder not working would cause a lot of other things not to work.  Depends on the cause of it not working though.

Time to invent railguns in 1492 DR!!!!!!!!!

Quote from Kotath >>

2) There is a divine edict limiting the power of GUNPOWDER (reduced to less powerful 'smoke powder') and you figure that Gond would allow nuclear weapons? Seriously? Any DM that would allow nukes to work would allow anything to work.... 

 Gods in D&D aren't omniscient.  So yes, Gond would allow nuclear weapons due to them being far enough removed from the technology level of Faerun that he wouldn't have any knowledge of them or understanding of what they did until after there's a glowing, radioactive crater (though honestly, an air-burst would be more effective for devastating a wide area).

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Quote from Motman >>

On Greyhawk they had high-tech weapons in the Expedition to the Barrier Peaks adventure so some advanced stuff works.  Never had anyone try to use gunpowder there though.  It was always magic and swords for my players.

Magnetic linear accelerators would probably be about the only way they could do it, but gunpowder not working would cause a lot of other things not to work.  Depends on the cause of it not working though.

 I think that the assumption was that Barrier Peaks was so high tech that no one would be able to reproduce the high tech, plus it was a crashed ship, so mostly non-functional (ignoring the existence of wish spells, etc) 

 Correct: characters were supposed to assume that the power armor, robots, and laser weapons were all magical items.  They were technologically so advanced that there simply wasn't any means for characters to figure out how to replicate them.

could a conjurer wizard just summon a piece of the sun? 

Quote from OboeLauren >>

The brute force way would be to cast Control Winds and send all the ships in the harbor crashing into the city.

The really nefarious way would be to cast True Resurrection on someone super powerful who'd be inclined to destroy Waterdeep. Like one of the Manshoons.

The fun way is to Gate in a swarm of Abyssal Chickens. ;)

 I vote fun way.

Overrunning the biggest city in the Forgotten Realms with infinite Abyssal Chickens certainly has some appeal.

Abyssal chickens aren't the most powerful demon, but certainly the most amusing. 

I vote to summon orcus, because his wand can create hundreds of zombies.