D&D 5E Give my Dragon Encounter More Bite!

[Cernor]

Thats a probably a better simulationist answer, but even I think that might be stacking the deck too much for the dragon. Give the party some chance to rescue the poor caster!

I suppose that dragons being enormous flying magical lizards is enough of an advantage over the glorified apes that call themselves "adventurers"... On the other hand, simulationism leads to wonderful fun like dragons divebombing a party and crushing them with its momentum (dealing quite a bit more than 20d6 damage). Or even better, the dragon grapples the caster, flies far up in the air, then divebombs the party caster-first! Not the kindest way to return the caster to the party, but you can't say it wouldn't be effective.

 

[Tormyr]

I suppose that dragons being enormous flying magical lizards is enough of an advantage over the glorified apes that call themselves "adventurers"... On the other hand, simulationism leads to wonderful fun like dragons divebombing a party and crushing them with its momentum (dealing quite a bit more than 20d6 damage). Or even better, the dragon grapples the caster, flies far up in the air, then divebombs the party caster-first! Not the kindest way to return the caster to the party, but you can't say it wouldn't be effective.

It is what my dragon would do. Fly in, grapple the caster in place of a claw or bite, first opportunity, use wing buffet to knock pesky fighters back and take off. Next turn, fly up to 90 feet, drop character, return to 60 feet, fire cone breath attack straight down.

So many options...

 

[Superking]

In my Thundertree, the players encountered the dragon who wanted the PC's to get rid of the cultists. Venomfang accused them of stealing her eggs and they were using the eggs as blackmail as blackmail.

The PC's then came upon the cultists in their makeshift HQ, who where trying to parlay with the dragon and wanted the PC's to help in their efforts. Their plan was to rid the town of the twig blights and zombies and in doing so would appease Venomfang.

The PC's sided with the cultists and began clearing the town. One of the PC's searched the HQ of the cultists for eggs and eventually discovered cult plans sent to them from Rezmir (this linked the end of LMoP to HotDQ).

The final showdown had the cultists and the PC's approach Venomfang, the cultists being cultists turned on the PC's in an opportune moment and offered them as homage to the Great Venomfang. (spoken in Draconic) and the fight was on. Pc's vs. Cultists outside the town square with Venomfang watching on for a round. Then she jumped into the fray. It was an epic fun battle.

One twist I added was a statue in the town square being that of one of the PC's Grandfather, who was a renowned dragon fighter in his day. So the showdown was fought in the shadow of the statue figure. I offered small bonus of advantage here and there if that PC was near the statue.

That is how I amped the encounter. Through in cultists and the dragon at the same time!

 

[Jaelommiss]

Grapple with first claw attack, grapple with second claw attack. Next turn, spend action to rip the pesky wizard in two.

 

[Psikerlord#]

To elaborate on what some others have brought up:

A solo CR8 dragon is a deadly encounter for 4 level 4 PCs. For 6 however, it is only a hard encounter. To have been a deadly encounter of the same magnitude for the 6 character party, the dragon would have had to be equivalent to a CR12 creature. However, you want the increase to be solely through increasing its hp. Increasing the dragon's hp to 289 will adjust the dragon to CR12. At that point, the PCs can attack as much as they want and the dragon will give as good as it gets.

However, now that the solo encounter is over, there are all the other good suggestions that people have put forth.

I would go with something like the above - bump Venomfang up to CR 9, 10 or 12? (she has increased in level, too!) and give her legendary actions (3 points?) and legendary resistance (2 points?). And possibly "lair actions" and flesh out some cool lair powers. This will make her a viable threat all by her lonesome, especially if also played intelligently.

Without legendary actions few solos are a match for a whole party, even with much higher CRs. As for fluff to explain this increase in power... ermm... hmm maybe she has located a new lair based on a ley line. Or dragon cultists are feeding her power somehow. Or she's just had a lot of fighting recently. I guess it doesnt necessarily need explaining...? I would foreshadow the increase in power however, make sure the PCs know this is not the same dragon as before.

Alternatively... as many have already suggested, add a bunch of minions to help. Dragon cultists maybe.

 

[Sword of Spirit]

It is what my dragon would do. Fly in, grapple the caster in place of a claw or bite, first opportunity, use wing buffet to knock pesky fighters back and take off. Next turn, fly up to 90 feet, drop character, return to 60 feet, fire cone breath attack straight down.

So many options...

I think the official ruling is that you can't replace part of a multiattack routine with other sorts of attacks like shove or grappling, as you could with a single attack. I might change that for another campaign, but I'm making very few changes to standard rules in this one. I definitely considered having the dragon pick up characters for the last fight. Since moving a grappled creature reduces your speed by half if you are only one size larger (and realistically, if they are struggling and squirming around it kind of makes sense), she would only be able to make a single grapple, and then fly 40' in that turn. Positioning to avoid attacks would make it really hard to get in an get out. Basically she'd be within two or three dozen feet of the target location for two rounds (which is bad news unless she's picking off a lone target with no friends within' 60'), and probably only be able to drop them from maybe 30'. Even dropping the target on top of someone else is looking at doing a max of 3d6 to each (I'd actually have the person on the ground take half, Dex save for none). She could potentially do twice the total damage by just flying into melee with that person and multiattacking them. I really wanted it to work, but I eventually decided that you just can't make it effective with a Large dragon. A Huge dragon would be another story...

 

[Cernor]

I definitely considered having the dragon pick up characters for the last fight. Since moving a grappled creature reduces your speed by half if you are only one size larger (and realistically, if they are struggling and squirming around it kind of makes sense), she would only be able to make a single grapple, and then fly 40' in that turn.

This is why we have a DM: to screw with the RAW to kill the PCs. It makes far more sense that one of the dragon's attacks would be replaced with the grapple attempt (like a PC), and makes for a tougher fight. As I see it, the speed reduction when grappling is because the grappled creature can still brace itself against the ground and attempt to counteract your movement, but a creature that's flying has nothing to brace against to stop the movement.

Besides, it makes for a better story if the dragon can move at full speed while grappling and replace attacks with grapple attempts. Without them dragons are weak (well, considerably weaker), and if you want weak dragons, you can go play Skyrim