D&D 5E How many (ancient) dragons would it take to destroy a (dwarven) city?

[WayneLigon]

A lot depends on the setting. The phrase 'mithril golems' really threw me for a loop because in my mind, there might be at most one such thing in the world, not acting like guards for a city, even a city of dwarves. They certainly don't have armies of Iron Golems. Perhaps if this was Eberron, but not the 'normal' sorts of D&D worlds I think of.

To my mind, a huge ancient red dragon utterly decimates a dwarven city. Or any city, really. Almost all of the city defenders drop their drawers at the Frightful Presence, and get in the way of those very few who do not go mad from fear, hampering any possible response - you've got a handful of thousands of trained warriors (and this only because dwarves are traditionally a very martial culture), trying to wade through screaming mods of tens of thousands. Never underestimate the power of a mob of people literally insane with terror. For a human city, I'd say that in and of itself would be enough to effectively destroy the place. There might be a literal handful of people in the entire city even capable of dealing with such a threat (ie, PC-level characters). It rips open the main gates like an anteater with a termite mound, and devastates the place with it's breath weapon, melting stone and shattering buildings.

 

[aramis erak]

Working strictly with averages...

Given that it takes approximately 400 common men① to do about 1d6 damage, and that requires long range missile fire to attain.
Within 120', one can get 400 common men in by having roughly 1 per 4 squares.②
The dragon has a 90° cone breath weapon, averaging 80 guys killed per 3 rounds. ③
The dragon has a 15' radius on the wing beat, and kills about 8 more with that every round.④
The dragon has a tail attack as a legendary. It kills one more every round.⑤
On rounds it doesn't breath, the dragon can kill as many as it can reach with the multiattack Claw/Claw/Bite, killing about 3 per round. ⑥

So, the attack pattern is
1,4,7,11, etc: Breath/Tail/wings, for 87 80+1+6 (because two of the guys in reach died from the breath)
2,3,5,6,8,9,12,13, etc: Claw/Claw/Bite/tail/wings for 3+1+8=12

And the dragon takes 1d6 per round. It takes 75 rounds, and thus 25*(87+12+12)=2775 common men, motivated to do so, to kill the dragon.

The ancient red dragon needs about 150d6 of damage to be killed. It probably won't stick around if it's starting to look dicey; they're proud and arrogant, but hardly stupid.


① A DC26 save is failed by common men. This means they have disadvantage. To hit AC22, they need a need a nat 20, as they don't have a positive modifier from skill, so it doesn't matter if they're proficient or not. So, with disadvantage, a nat 20 is 1 in 400. Since you have to be proficient to fire at long range, and they're not, you give them all longbows and pray.
② 120' is roughly 1810 squares. to get 400 men in there, that is 1 per 4.5 squares, round down to 1 per 4.
③ 90' cone is 324 squares, or 80 guys at 1 per 4 men. 5-6 chance of recharge means an average of one breath every 3 rounds. It's save for half, so common men die either way.
④ 15' radius is roughly 29 squares, or about 8 guys killed. the DC23 cannot be made by common men, and the average damage of it is more than double a commoner's HP of 4. Dead.
⑤ Average damage kills outright.
⑥ 2.8, really. 19/20 chance, doing a minimum damage enough to kill them.

 

[Rod Staffwand]

What's past is prologue.

A dragon can take out a city of 50,000 dwarves if the adventure says: "A hundred years ago a dragon took out a city of 50,000 dwarves."

*Maybe the city was at the center of a peaceful kingdom in a peaceful era and they grew complacent about their security.
*Maybe the gates were opened by the vengeful dwarf prince who was past over for the throne and whose ghost now haunts the city.
*Maybe the city unwisely relied on natural gas lines to light their cavern city--flammable natural gas lines.
*Maybe the dragon turned the golem defenders against the city.
*Maybe kobolds...lots and lots of kobolds.
*Maybe these dwarves once crafted a suit of armor for a gold dragon that made it impervious to all damage and the dragon slipped into the city, stole it from the treasury, put it on, and became unstoppable.

All you need is some plausible reasons (preferably ones that add flavor to the adventure) and say that's what happened.

Trying extrapolate dragon v. dwarf city violence from a set of combat rules designed for adventuring parties is an exercise in futility. Beyond the scaling issues you have to contend with far too many unknown variables to make it worthwhile.

Thematically, ancient dragons are city-scale threats. That's all you really need to go on.

 

[Dr. Strangemonkey]

Short Answer: 1, if it's clever and the dwarves have at least mild competency issues, which isn't totally implausible particularly if the dragon has aggravated them through cursed gold and such. Still 1 requires a story or scenario and, obviously, limits the scope of the problem.

Long Answer: 50 is a fair number of thousands for a pre-industrial economy. That's a lot of infrastructure. If it's a mining culture that's going to expand that base by a lot. That's a lot of chances for to go wrong for said dragon, even if only on the level of inconvenience and on the logistical side. The goal here isn't for the dragon to survive but to conquer, it's going to want help. Plus, you get some more dragons in on the action with associated minions and you've got yourself either some mini-bosses or a posse to ride back in and take revenge whilst the PCs are relaxing with the horde.

So 2+, a small family of chromatics, a motivated cabal of them, or whatever along those lines gives you more options and takes it out of 'this is a freaky incident' to 'truly this is the nightmare of our culture' territory.

 

[MarkB]

Personally I wouldn't make it a stand-up fight. There's no great story to that, and it only shows off the dragons' brute strength, not their cunning. The dragon should be using its knowledge and capabilities to visit some great calamity upon the city, leaving its occupants in not fit state to fight back.

For a black dragon, I'd have it start upstream of the city, blocking off the aquifer that supplies most of the dwarves' fresh water. To make it look like standard siege tactics, it also attacks supply caravans and razes outlying food production areas. But meanwhile, once there's no outflow to give the game away, it's quietly blocking up all the outlets downstream of the city, and breathing poison into the ever-growing reservoir of water in the blocked aquifer.

Maybe it unblocks the aquifer itself when it's ready, or maybe it takes the crueller, lazier option and allows brave bands of dwarven sappers to sneak and fight their way to the blockage, so that when they finally clear it in order to regain access to water, it's the dwarves themselves who unleash a torrent of toxic filth upon their city, which steadily floods it until only a few dry, clean areas remain. And then the dragon finishes off the survivors at leisure, using the vast poisonous lake filling most of the city to full advantage.

For a red dragon, maybe a similar approach but involving lava instead of water.