So, I'm DMing Hoard of the Dragon Queen for some friends who have been wanting to play for a while. i had them start at level 3 since they would be able to stand up against the sacking of Greenest much better and wouldn't be knocked unconscious by the first nat 20 I would roll. I had made Greenest inro more of a walled city that would showcase the destructive power of the Cult and so they would experience how much of a threat the cult was. We had a Verdan Monk (Way of the Open Palm), a Human Paladin (Oath of Conquest), a Human Rogue (Thief), and a Dwarf Cleric (Forge). I gave them some backstory pf how The Order of the Gauntlet wanted them to scout the area the Dragon Cult had been operating in and report back to their commander. As they made their way to Greenest, they saw Lennistor, the Ancient Blue Dragon, destroying many parts of the city with his lightning breath (I know he'ssupposed to be an adult, but I wanted to experiment with him as an ancient). The characters decided to rush to the rescue and fend of the great evil. So, the characters fought their way to Greenest Keep, some falling unconscious to kobolds and cultists along the way, but the cleric and paladin made that pretty inconsequential. They arrive in the keep and immediately begin a short rest… except the cleric. I'm thinking "ok well maybe he's at full health or very close to it, no biggie". Then the cleric asks if there's any weaponry on the walls like catapults or ballistae. I knew that the dragon fight was happening soon, so I decided to give them a little bit of a chance and gave them two ballistae, saying all other equipment was too beat up to use, but I gave the stipulation that the ballistae were out of ammo. The dwarf then said "I'd like to spend the rest making ballistae ammo out of the other equipment". Ok… how many do you want to make? "I'd like to make 10" Will anyone assist? Most of the party decides to assist and they end up convincing a lot of the guards and injured people to help gather materials and they made 10 pretty shoddy ballistae bolts, 5 for each ballistae. As a DM I knew they wanted to use them against the dragon, so I gave them the opportunity. Lennistor lands in the castle's courtyard and initiative is rolled! The Dwarf rolls a nat 20….. as does the monk. The Paladin and Rogue roll below 10 and Lennistor rolled about a 15 or 16. Ok, I'm thinking that they got a couple rounds to hopefully land a hit and not be destroyed by lightning or any other of a dragon's weapons. So the Monk goes first since he has a higher dex score and mans a ballistae. "I'm going to aim for it's wing!" he says to me, excitement in his voice. Ok, roll to hit. "NATURAL 20" he's jumping up amd down, eyes alight. (I have a rule where if you get a natural 20, you can describe your attack and what happens as long as it's within reason and it won't instantly kill a creature as long as its HP is high enough) Alright, I say, what do you do? "I want to pin its wing to its body. Oh no, I think, they're dead meat. Its the Cleric's turn and he mans the other ballistae and you know what? He rolls a nat 20! So he decides to pin the other wing! Ok, now its Lennistor's turn. He's going to use his breath weapon on the first ballistae. Monk roll saving throw! "Uhhhhh….. 21" Ok, he makes it, half damage. I roll all ones and twos so he only take like 12 damage…. much to my dismay. The Paladin and Rogue decide to assist the two on the ballistae with reloading so the can fire next turn. Top of the second round and they both again…. roll high enough to get past a 23 AC. This goes on for about 5 or 6 rounds and I'm rolling so horribly. I've gotten natural ones and twos and threes on every single one of my attacks as Lennistor and since the party is working extremely well together and have immobilized the dragon to the point where it's not even fair any more for the dragon. They, successfully did about 500 damage give or take with them rolling at least five natural twenties through the entirety of combat. The dice gods loved them and smote me to dust.