Preface:
We ran a test game of this in 5e which proved to be the best version to run it in. During the test game, the mayor turned out to be the bad guy. This game was that but version 2.1.
We enter the town at night, and with the exception of me, everyone is a government sanctioned official. (I am a merc the group picked up who is nominally on their side.) We are there to investigate paranormal activity which we are 99% sure is a Dullahan.
Party Composition:
Me - Drow Mercenary (Chaotic Neutral)
Max - Human Gunsmith (True Neutral)
Greg - Half-Elf Scribe (True Neutral)
Stefan - Human Huntsman (Neutral Good)
We meet the mayor, who invites us into his private carriage and has his driver (just some ignorant peon who was in the mayor's service) take us to his mansion.
The Situation Now:
I suspect the Mayor of fucking us over, so I ping the Max, who was the party leader, that I am going to trade places with the driver. This way, if the Mayor does fuck us, I can just pull the carriage over and stop it. Max agrees, as he has a plan in action for dealing with the Dullahan.
My method is simple: Ping the driver with Message cantrip.
Me: I say, "THIS IS THE VOICE OF YOUR GOD, PULL OVER AT ONCE."
DM: Alright, roll Deception
Me: Cool, I'm proficient in this as well. Hooray for the Criminal background!
>All my "d20, go!"
Nat 1.
The driver was an ignorant peon raised in some backwoods town. Upon hearing this, he freaked the fuck out as we go around a turn at high speed. The carriage zooms off the road and hits a ditch which is about at a 75 degree angle. You can guess what happened next.
I am catapulted from the roof and forced to make a DEX saving throw. Obviously I succeed, as I am mechanically a Drow Shadow Monk. I land stylishly, almost like Spider-man, and hear the carriage impact the ground behind me immediately followed by screams of pain and a gunshot or two.
Casualties (in order):
-The driver goes under the wheels but doesn't die from this. He miraculously survives with like 2 HP.
-Stefan is thrown out the door, taking an assload of damage but survives (mechanically he's a Ranger, woop woop for that d10 HP)
-The carriage pancakes the horses into oblivion
-The Mayor breaks his neck, being killed on impact
-Greg and Max are thrown forwards, but Greg lands on the Mayor's body which cushions the impact
-Max lands on his blunderbus, causing it to discharge out the window and hit the driver, killing him
Upon witnessing this, I immediately run over and cast a spell that let's me steal the Mayor's face. I then burn the carriage and the bodies to cover my tracks and lead the group to the mansion in disguise as the Mayor. I waltz in through the front door and I am greeted by servants ready to do my every bidding.
After I exploit the shit out of the Mayor's position of prestige for a day or two, the town guard manages to find and identify the Mayor's body. They rally their men and assault the mansion. They smash through the front doors with intent to announce their presence, their plan being we surrender and they investigate.
Unbeknownst to them, however, Stefan had a trap placed in the front door in case the Dullahan came through. They catch a Cordon of Arrows to the face and now any intent of being merciful is forsaken.
After they sweep through the mansion, they kill Stefan and me (me getting the worst since I'm a Drow), Greg manages to escape with the Suggestion spell and Max sets the mansion on fire with the help of Greg and a Fireball spell.
The bad guy, however, was one of the guards. This sounds like a good thing, so why the "however"? Because the monster wasn't a Dullahan. It was a Devourer spectral demon being contained in a host who happened to be the captain of the guard.
This thing is now unleashed upon town causing havoc at every turn. We also notice that the Mayor was a novice alchemist and had an apothecary containing a powder we would use to stun the Devourer, too. So there is that.
The resulting fight is bonkers. Greg cuts off an elf's fingers for a banishment spell, Max takes up position in a church tower with his blunderbus to provide covering fire. Another player jumps in as a Druid and uses Conjure Animals. The nutcase then charges the Devourer on a rhinoceros to buy the group time (he gets rekt by the Devourer, though).
Max runs dry on ammo, using an NPC as his replacement weapon while Greg conducts the Banishment spell. The NPC, however, is a Warlock who's fiendish patron is on good terms with the Devourer. The Warlock passes harmlessly through the Devourer, which is about to kill Max as Greg completes his Banishment spell.
After the Devourer is gone for good, Max kills the Druid and Greg for fucking his plan up. He then leaves back to his headquarters in the central government.
And that is the story of my crit fail being directly responsible for a party wide death.
