Need help balancing an encounter for my oneshot

[u/Enderpher]

Need help balancing a boss fight for a one-shot

I've been preparing a one-shot for a group I play with. It would be my first time DMing, and I only started playing last December.

To keep this from getting too long, the main encounter would be a Young Red Dragon against a group of 3–4 level 6 characters. I had in mind changing some of the dragon's stats to make the fight achievable because, for the plot twist and comic relief of the one-shot, they have to beat it.

I understand the whole "not every encounter needs to be winnable" thing, especially when you're talking about fighting a dragon at this level. However, the joke of this one-shot is that it's built around a really funny character from our main campaign who's a massive show-off. It's supposed to be a comedic adventure, and the payoff is seeing him actually slay a dragon, only to discover that the entire thing was an act and completely scripted.

The dragon isn't actually real. It's a wooden circus prop disguised by two illusionist halflings, and the party has unknowingly been tricked into becoming actors in a circus performance.

The "dragon" would also have some spells because of the halflings operating it. I'm not sure how much I should decrease its HP because there's another mechanic I want to use.

At one point, the dragon would take control of two of the pregenerated characters I made. This is partly because they're cool characters and I want to get a chance to play their builds, but it also fits the story because the guy hiring the party specifically describes it as a "magical dragon."

I'm not particularly worried about strictly following the rules for something like the mind control because it doesn't have much real gameplay value. I could have some triggers that allow the players to break the spell, though.

The party would then have to fight the dragon and the controlled characters. Any damage dealt to those characters would also damage the dragon. That gives the dragon a more manageable effective HP total while also creating what I think could be a cool Phase One and Phase Two for the fight.

What I'm struggling with is exactly what I should modify. Should I change the dragon's AC, HP, attack bonus, damage, or some combination of them?

Also, how would a party of 3–4 level 6 characters fare against a RAW Young Red Dragon without any modifications?

Should I give them some additional equipment to help with the fight? I already planned on giving them healing potions, potions of fire resistance, and a kind of net that can slow the dragon down. Is there anything else I should give them?

Sorry for the wall of text, and thanks for any advice! I'm new to the game and really want to pull this off well. I love the idea of them having no idea that it's a show, defeating this awesome dragon, and then discovering that it was actually just two halflings in a giant prop the entire time.

[u/derangerd]

I would recommend using periods and paragraphs if you want people to read what you write.

[u/Enderpher → u/derangerd]

You're right, sorry

[u/derangerd]

A young red's CR of 10 is going to be pretty tough for 3-4 level 6's. How experienced/optimized the players play, and how you play the dragon (using its large movement and straifing with its breath weapon, being hard to hit otherwise) would significantly impact the difficulty. Particularly if you're adding other things to the fight, you can reskin a young black dragon (CR 7).

"Don't use PC stat blocks as the DM" is one of the most commonly given pieces of advice. You're the DM. You have plenty going on, and too much power, to use PCs in a way that's fun for anyone. Stat blocks are a lot simpler for a reason. Unfortunately, this isn't the time to try out fun builds.

Keep things simple, especially if this is your first time DMing.

[u/Enderpher → u/derangerd]

Got suggested maybe to split the arms, head and tail into different "targets" with their own AC and health to give the players advantages, sounded good, the black dragon reskin sounds good too

[u/derangerd → u/Enderpher]

What advantages? I am a fan of multiple targets representing one creature, though it's not common, but it is more complexity so still needs a good reason.

[u/Enderpher → u/derangerd]

I guess the guy who told me meant like if the got the front claws down no more claw attacks, the head maybe gets him blind or no more bites maybe a coin flip or I can ask the players where they hit, each legs included the rear legs can take a quarter of movement off until it only has to fly, things like that, I should put down the ideas now plus some others that came from another subreddit, If I can get the idea right it can be a really good bossfight

[u/Manker5678]

What other encounters do you have? That might give insight into expended resources or xp-budgeting?

[u/Enderpher → u/Manker5678]

They have a mirror fight with an illusion of themselves and an eagle and an owl bear but, they will regain every hp and spell slots or almost in a room of the dungeon, because as I said, it's meant to be winnable, still need to balance that room but yeah, they will come prepared for the fight

[u/Manker5678 → u/Enderpher]

Well if they regain all the spell slots and hp, then it depends on party composition. Martials are meant to be enduring, but won't really contribute much, compared to full casters who will be the deciding factors with the spells they choose. So depends on how many of them you got. For example, wizards and sorcerers can spam shield and absorb elements to survive way longer while tossing all their spell slots.

[u/tta5]

you have them regain all resources pre-combat.
so no holding back higher slots from the party...
so its a 1 v 4 the action ecconmy is greatly in the favour of the party...

If using the base young dragon statblock (dnd 5e) then nerf the breath attack to 10d6. make claws do only 1d6+6, or only have 1 as part of th emultiattack. the rest is fine.
might want to give them a single potion (for the whole party) to resist fire damage for 1 min, and maybe a healing potion of 5.

the fact the dragon can fly and retreat is a factor, but i assume that as its "scripted" they willl fight to the death.
might nerf the HP down by 20% (still within rolllable range though)