My players were at the last section of this particular area, with one more encounter to go...a fight with a young dragon.
There are a few RP encounters before the fight, and I'm just super fucking glad they did what they did. Instead of just murdering everything, they decided to pass off as fellow members of this organization that was trying to curry favor with the dragon for their needs; offering gems and two prisoners as tribute which just so happens to be two PC characters that aren't being played anymore (they got kidnapped ages ago).
Long story short (future me here saying HAHAHAHAHAHHAHAHAHHAHAHAHAHHAHAHAHAHAHAHAHAHAH), they converse with the dragon as a big group, however, the players, mid-negotiations, reveal that they are not and try to convince the dragon that they are not there to pay homage to the dragon but to kill him, in which the leader of the other group denies it. So now, I had to make a choice here. I could have had the two groups fight while the dragon watched, pleased at the entertainment, but then that leaves the group for very little options afterwards other than, okay now time to take down the dragon because, regardless of the prior actions, the dragon isn't going to leave the area (which is the entire reason for them doing this) which would end in a fight with just the party and the dragon right after the fight with the other group and that could have been interesting and neat but would have most likely ended with death.
The other option was joining forces, despite the situation and everyone's allegiances and motivations for being there, and killing the dragon together. This way I can still make the dragon bad fucking ass as it rips apart random NPCs and just being a more "normal" threat for the real players. I rolled quick for the leader and she suggested that everyone band together to take down the dragon and deal with all the other shit afterwards and luckily they agreed, so now it's 11 characters against 1 young dragon which seems much more reasonable.
Everyone does initiative and I plan to do something big the first turn; a huge display of OH FUCK to really hammer home how dangerous this whole situation is. My goal was to obliterate most of the chaff NPC's to up it's OH GOD NO WE'RE ALL GOING TO DIE factor while doing some damage to the players. I remember reading the skill and thinking "Well, at least they can roll to save and it is possible to roll badly and it's written this way in the actual encounter so they know better than I would.
The dragon goes last. Everyone does their first turn and do a tad of damage but at least people are hitting the thing. So far so good, I guess. It's the dragon's turn and I start with the flavor text (first mistake), inhaling as if to ready a breathe attack, and have them roll their constitution saves (second mistake), and finally, roll out the damage (third mistake). It was a little hilarious picking up 6 d6's, the monk yelling out OH MY GOD THAT'S SO MANY DICE, and I think in my head there are 6 more coming, oh god there are 6 more and I'm not rolling 1's; I'm getting a bunch of 6's and 4's and 3's and then I pick up the six again (fourth mistake), my brain is saying no this is already terrible stop it here, just stop it here but I really wanted to know what the damage would have been, I really wanted to see what kind of destruction could be had, forgetting just how squishy everyone really is. I rolled them again to more 6's and 4's and oh look, a couple 1's, like those matter now.
It literally killed everyone. Wait, no, sorry, it would have but the monk was standing next to the dragon when he did it so, you know, he wasn't in the cone of death but everyone else fucking died and the monk would have died the next round anyways.
So now I'm sitting there just thinking I can't do this it can't end this way I'm pretty sure they want to keep this going and while yeah, danger and the threat of death is needed but this was just fucking ridiculous and kinda my fault and everyone is worried that it's all over and I just flat out say, no. I could have been secretive about it, but because I gave all the flavor text about the Breath and the con saves I couldn't just say OH WAIT INSTEAD HE JUST TRIES TO BITE YOU I mean that's what I ended up doing, it's multi attack is fucking mean as it is so I back pedaled and said we weren't going to do that and instead changed it to multi attacks instead. I change the flavoring to it was about to Breathe Poison, but it was still too young of a dragon to do that, or it just drunk all the booze at the ruined tavern and couldn't make it work and yeah, that did get worked in because I was trying to RP a fucking dragon and I couldn't say "Draconic" because I'm a blustering buffoon so I made an off comment about that and that's one of the reasons why he even would have people disturb it's peace, even with a tribute ANYWAYS, THE DRAGON CAN'T DO THAT so it did other things.
At first, having 11 characters hitting the dragon, it was kinda looking like an "easy" fight with the group taking the dragon down to nearly half HP rather quickly. The dragon got a few good hits in to some of the chaff, which the PC's got only some scrapes, really only the monk. So it went from an impossibly hard fight to, unbeknownst to the players, a relatively easier one. It didn't help that I was rolling good for the 6 NPC's and not so good for the dragon, but again, with the knowledge that they were spared from everyone dying kept the tension up, but still, I wanted it to be interesting and exciting. I finally rolled well on a bite and claw and took out two NPC's in one fell swoop, with the bite being extra grisly, but my thirst for death wasn't sated yet. They were hitting it too much, and it was really close to dying, like, actually dying, not the point where it could fly away. Nope, that was long gone; decided this dragon was young and stupid and filled with hubris and got over it's head so it's staying till the end, but the end was coming too quickly noooooooooooo it has to do something coooooooooooooooooooooooooooooooooooooooooooool.so I had it use Poison Breath lol. Granted, I halved the dice amount and lowered the cone of death from IT HIT'S EVERYONE to IT HIT'S THOSE TWO NPCS OVER THERE AND NOTHING ELSE and hilariously enough, even with less dice, they were still instantly vaporized. Having less of these NPC's will also be helpful afterwards; if they survive I want the players to have the "upper hand" in the next bit that could lead to some neat RP moments.
I was also super greedy because the dragon actually died but I gave it more hit points just so I could nearly maul the monk to death. That was pretty cool though; dragon picked him up with his mouth and the monk getting free but poisoned and left with 5 HP. Okay I'm good this thing can die now in a cool way next round oh shit wait the Ranger is moving toward the dragon DON'T GET AN OPPORTUNITY ATTACK AGAINST YOU SHIT GODDAMNIT didn't know it's reach was 10 ft. instead of 5 ft. I guess you get bit too and now the Ranger is nearly dead. Thankfully, the monk ended it in a very neat and satisfying fashion and everyone rejoiced. They ended it there and will pick up with the other survivor NPC's next time. Everyone said they enjoyed it, too so, that's good. They had fun regardless of how botched it was.
I did fudge a roll though that I feel bad about. The Wizard wanted to shrink the dragon and I failed the roll, but because I didn't know how it worked exactly and this was at a point where people were still not vaporized and I didn't want the fight to be that easy, if it would do what they wanted it to do, I re-rolled and saved, and the fight went on. It wasn't all a wash though; the wizard is now super excited about the utility of it alongside the spells that just make things dead and it's rather exciting because people are finally thinking about other possibilities except for this one time I'm so sorry I just wanted to eat someone and rip someone asunder and melt people in poison I'M SO SORRY I FUCKED IT ALL UP.
So yeah, group and co killed a young dragon and everyone had fun and all was well, except for that one part where they all died to the dragon.