So, you have to destroy waterdeep with one spell, what spell do you use, it can be any 5e spell form any source and assume you can cast it at any level, what do you cast, and don’t be afraid to get creative! (This is not a player looking to do this just a fun hypothetical) also no casting wish, that’s the easy way out

edit: you can say ideas using multiple spells just nothing like “spam mass suggestion”

I'm not exactly sure what you're looking for, but the idea of Thunderstepping everywhere until the city is in ruins is rather hilarious.

One Spell to destroy a city?

Gate, no question.  Gate in an army of demons.  Even just pulling in one big one will destroy a city.

If time is of the essence,  Storm of Vengeance has a 360 ft range and should kill everyone in the open or even in a wooden building.  But Waterdeep has lots of stone buildings.

Only slightly smaller, Tsunami gets an area 300 ft wide, 50 ft long, and moves upto 300 ft forward.    That ls a square 300 ft x 350 ft, that should take out stone buildings.

If you can cast any spell at any level, we're assuming that you're a 20th level character, right? If that's the case, you could simply use true polymorph on yourself to become an ancient brass dragon. From there, destroying a city should be no problem.

Quote from 1AmGroot >>

If you can cast any spell at any level, we're assuming that you're a 20th level character, right? If that's the case, you could simply use true polymorph on yourself to become an ancient brass dragon. From there, destroying a city should be no problem.

Waterdeep is protected from dragons by the dragon ward.

Wish

It works for anything.

To expand on the dragon ward comment- You’re talking about a city with many high level casters and at least one Demi-god in it....let alone all the temples that various dieties are watching.  I strongly suspect they’ve taken measures to insure some clown doesn’t come in and destroy them and everyone in it with the snap of a finger.  Even if not, I wouldn’t be surprised if a god just came down and said ‘oh hell no’ and then erased the offending character’s existence.

on the gate comment- assuming it could be cast within city limits- sure one large demon would do some damage-or an army-but very quickly you’d have an army of wizards, clerics, paladins, druids, etc, etc, opposing those demons and then they’d turn their attention to you. 

you’re talking about a very old city that has withstood a lot- including many many years of all variety of top-level characters thinking they’re the greatest thing since sliced bread and way smarter than everyone else in the city with totally unique ideas (like trying to destroy the city with one spell) passing through its streets -. No way it would get wiped out by one mortal.

could you do damage to the city? Sure, maybe even to mulmaster-damage levels.  Could you destroy it? Nah.

Quote from Fattsgalore >>

Wish

It works for anything.

 ‘it could potentially work for anything’. big diff IMO....but like with dragon ward, I strongly suspect you’d find a countermeasure is already in place to deal with that.

Assuming I could get it to stick, it would be hilarious to cast Geas on Halaster. Just imagine him sending the contents of Undermountain into the streets :-D

If I can cast at will with no limit on how frequently I can cast it, I'd maybe go with something like Mass Suggestion to get the residents to go into a frenzy and destroy their own city...

Yes but, I said no wish

You can only cast the spell once

Quote from Fattsgalore >>

Wish

It works for anything.

 OP clearly said "no casting wish".

Quote from DestroyerElf >>

You can only cast the spell once

 

 

 Then it's not possible.  Waterdeep is such a large city and has so many high-powered characters living in it that a single casting of a single spell will never come close to destroying it.  Especially in 5th Edition with the limitations placed on magic.  It might have been possible in 3rd Edition using Epic Magic, but even then it's iffy.

Make a deal with a devil, allowing you to cast infinite fireballs.

As others have said, Waterdeep is full of strong NPCs, so just throwing Evocation at it probably isn't going to work. You need to get the city to destroy itself. Turn those NPCs against each other. Transmute all the gold in the city treasury into mud to crash the economy and get the people rioting. Let them do the work.

Quote from scatterbraind >>

As others have said, Waterdeep is full of strong NPCs, so just throwing Evocation at it probably isn't going to work. You need to get the city to destroy itself. Turn those NPCs against each other. Transmute all the gold in the city treasury into mud to crash the economy and get the people rioting. Let them do the work.

oh-better way to crash the economy imo would be to transmute all the mud into gold.  if you go gold-> mud, gold becomes even more valuable.  if you go mud-> gold, currency itself goes out the window. the 'haves' realize they have not and people start trading in sheep.

There is no official spell, feat, class feature, subclass feature, racial, subracial, or anything such as that can transmute mud into gold or vice versa. Not even the transmuter's 14th level feature.

Also, HeathSmith has a point. Gold is given value by rarity, so everyone having lots of access to gold means they'll give it little value and use something else instead. As a relevant factoid: copper used to be significantly more valuable than gold - until we found a way to get a lot more of it, and just like that it's value dropped greatly.

When I suggest Gate, I was thinking a better army than just Abyssal Chickens.  Though I do like a creature that is so bad at flying that it automatically falls if it ends it's turn in the air.