Rebuild and Tactical Help with Dragon?

[BrokenChord]

Next session, my players are going to be entering a town where an Old Black Dragon lives. Here's a quick overview of the party:

All 12th level:

An Elf Urban Druid with a Medium Animated Object and Wild Cohort-acquired dire bat as ACs who usually uses healing and buff spells;

A Halfling Wizard with a Young Copper Dragon as a mount who focuses on blasting spells and transmutation utility spells;

An Orc Knight/Fighter who uses a guisarme, mostly prefers charging and Power Attacking but also has Improved Trip for whenever the group is forced into the defensive;

A Merfolk Dragonfire Adept who wears Mithral Twilight Nimble Feycraft Breastplate and basically huddles there in her armor using invocations and, when possible, her Enlarged Line of Lightning breath weapon;

And last but not least, an Orc Favored Soul who almost never shows up and used a homebrew feat to get the wings early, focused on blasting, but is currently 3 levels behind. Best not to include him in your calculations.

So, yeah. There's them. Keep their abilities in mind; I need to present a good and preferably lengthy challenge with this fight, as it may be the only one this (8-hour) session given where the plot is headed at the moment.

Who am I kidding, my games don't have a plot.

So, yeah. I'd like to build this dragon to be a lean, mean, city-fighting machine. The city is a large bustling business-y place, a lot like your average downtown business district with office buildings, apartments, and a few skyscrapers. I was intending to use the differing heights in the buildings to the dragon's advantage, doing things like popping behind a skyscraper and dropping down to street level to hide behind the lower buildings as it flies closer again, but I'm not sure yet.

How should I build this dragon in terms of feats, spells known, and equipment, and how should I utilize its tactics, to make it a worthy but not TPKing challenge?

My main concern so far is that the Orc Knight will feel useless; I'm considering dealing with this by having the dragon perform a sort of fly-by grapple to carry him to the top of a building, so they can melee it out for at least a round or two before the others catch up. Then I won't necessarily be playing the dragon stupidly (why the hell would it land for melee combat surrounded by his allies?) but I don't have to obviate the character for the fight.

In case it comes down to this, I'm willing to lower the dragon's actual combat effectiveness a bit if it lets him make more use of the urban environment, but hopefully I don't have to do this much.

Any and a help is appreciated, as long as it's actually an effort to help and not a joke post giving me advice to up the dragon's age category and optimize the hell out of it to murder everyone while obviating everyone completely "because it's what a smart dragon should do." I get that they're superbly intelligent, and to a degree I'm taking advantage of that. But I'm not particularly looking for an unfun TPK where everyone feels useless, as that situation is generally something I consider a failure in my DMing.

Not saying I expect most of you to do that, just, I've seen it and I'd like to avoid it.

Not that anyone who would have done it anyway will be stopped by my asking them not to.

Thanks in advance for teh halpz!

[VoxRationis]

Tear down chimneys and posts to make barriers, splitting the party.
The dragon has the speed advantage but the numbers disadvantage; it would probably flee repeatedly, trying to lure the group into separating based on speed while letting its breath weapon recharge. This presents a way for the players to outwit it as well; since the dragon is intentionally trying to split the party, it won't question (or at least will be less likely to question) one of them suddenly being gone. The fighter can hide in a building, wait for the dragon to try to engage the softer party members, and then charge out to attack the dragon. (Obviously, this is something the players should figure out on their own.)
Spider Climb would be a huge boon for it, allowing it to move quickly and with agility in areas many characters can't follow (such as shimmying between two tall buildings out of guisarme reach).

[Kamin_Majere]

Technically just play it as a 600 year old creature and enjoy.

An Old Black Dragon can entangle everything with in a 100ft radius with Plant Growth, then drop an 80ft globe of darkness on them while flying around firing lines of acid to any that think they can fly around in the air with it.

It literally has the strength to push over small buildings on the party making them have to dig themselves out while those that didn't get caught in the rubble have to face something that will gladly grapple them fly into the air and drop them from 150ft-300ft in the air.

Dragons are terrifying creatures and if played correctly will rarely just walk around on the ground and let parties pummel them to death. Its smart enough to know grappling spell casters and breath weaponing melee combatants will generally allow it to win almost any encounter.

Honestly you have to play them at way less than their capability to allow most adventuring parties to deal with these things because they are immensely dangerous, unless the party is optimized insanely well or are built to kill dragons. With 9 feats to work with this dragon could easily TPK the group. And even if just going with the base SRD version of the dragon would be one heck of a challenge.

You really shouldn't have to much trouble making the fight challenging and memorable for the group. Just mix up what the dragon does and maybe even fudge the rolls a bit so it doesn't wipe them out. Also give it the surprise round but use it for the plant growth trick so it doesn't lead off will killing the wizard.

EDIT:
Just as a fun aside, remember this dragon is going to weigh ALOT so having it able to just ram through buildings or landing on them and the building collapsing is always fun and makes great cinematics.
I also realize I did almost opposite of what you asked, while not any optimization just play style to kill the party... so i guess my advice isnt all that good there lol But about the best thing to do i would say is get all the parties hit points and play the dragon pretty hard core until you've got them to about the 1/3rd mark all around and then let the dragons "arrogance" be its downfall so they can start actually applying damage to it. About the only thing i cant think of how to do is getting the melee character to feel useful, between attacks of opportunity from its reach and the sheer amount of attacks it can put out in a melee turn the melee guy is going to be hard pressed to not die.

[Vaz]

I'm going to suggest this be a CR14 opponent; a Mature Adult Black Dragon is CR14; it's immune to the Breath Weapons of the Copper Dragon (+18 Fort vs DC17 breath, and acid immunity). It's DR10 is enough to protect against all Guard archers, making Ballista's really the only threat (unless you want to make the Guard Crossbow Snipers and giving them a decent Dex (Dead Eye+Crossbow Sniper = +Dex+1/2 Dex to damage if within 30ft, and 60ft Sneak Attack; consider Sneak Attack fighters as guards, which can help plink off bits of damage each turn).

If you want to help out the party in this instance (so they've got the odd drip, drip of HP going down to keep morale up (as players)); this can be aided by having the party aware of the Dragon before (and giving the Wizard a knowledge check to discover that it's got DR/Magic) and having the party wizard access to a scrolls of Greater Magic Weapon which he can then enchant some arrows with. You can then taper this off by having the magic bolts run out mid battle (say 70% HP), followed by the Dragon flattening the archery positions (say 40% HP) and so it just being reliant on the parties ability.

The Dragon has a CL of 5, which means it can take 2nd level spells, or CL7 as a Loredrake, which is 3rd level spells. With 2nd level spells, however, you have Wings of Cover, which is more than enough to cause shenanigans against spells.

Its Breath Weapon is a Line, in a City fight, that's outstanding, just get down an avenue, and turn an entire street into a foul smelling ooze. However, if the party does split up, Shape and Split breath feats give you the ability to just rip shreds out of the party.

For an interesting start to the encounter, you could give it Alternate Form (http://dndtools.eu/feats/dragons-ebberon--112/alternate-form--3585/), and give it the City Magic, Disguise Spell, Invisible Spell, Deceptive Spell could all be used to to give the PC's an idea that something naughty is going down; rubbish attacking passers by, invisible gusts of wind blowing people about, spells looking like they're being fired from arrow slits down onto the street, etc. When the players identify who is actually casting the spells, they attempt to arrest him (or see the guard arresting him), and *boom* he transforms into the huge dragon. That would *waste* his spells meaning that you won't be too powerful.

Other than that, multiattack, power attack, recover breath, etc.

[VoxRationis]

Oo, don't forget Lingering Breath for use on those narrow alleyways, particularly if they're blind alleyways. That would give the dragon incentive to get in melee range: trying to pen the party in the acid-filled space for as long as possible.