I want to preface this story by saying: first, I am sharing this with the DM's permission; second, no one involved bears any hard feelings from the conflict that happened. In fact, we're glad it happened, as it was an educational experience and is a great story.
There's a bit of background to this story that is needed to really appreciate it. Our group plays 5e and we were all really new to the game. I was running a homebrew campaign of which we were nearing the end. We had been playing for a few months and the party had reached level 4. They all decided that they wanted to keep playing the same characters for the next campaign, so when we decided who the next DM would be, we worked together to transition the two campaigns. We decided that the reward for completing my campaign would be a free feat for everyone and everyone to level 5. Furthermore, the only player who didn't have any magic items yet would be granted one.
His character was a half-orc vengeance paladin named Todak. Todak mostly used his spells for shield of faith and cure wounds to keep us safe, so I decided to give him an offensive boost. I originally wanted to give him the Sun Sword but I thought a legendary item for a Level 4-5 character would be a bit much. I toned the attack/damage plus down to just +1, and replaced the radiant damage with a boost to his smites. His smite damage would be doubled. It took us a long time to discover what a mistake that was as like I said, he mostly used his spell slots for defensive spells.
The new campaign starts and its a lot of fun. A great story, a really cool world. We were loving it. But slowly, things changed. We were noticing that our paths got more restrictive. Things that should require a simple roll had near impossible DCs, and the encounters got crazy. We were all pretty buffed up so we could take them, but soon enough there didn't seem to be a perceptible strength difference between boss encounters and random thieves on the side of the road. Spells just sometimes didn't work, and if they did enemies would almost always pass their saves way over what they needed.
I started to get a little frustrated, but I understood that we were higher level than earlier so it was going to be a little harder. But I didn't like feeling railroaded, encountering random thugs with as much HP as dragons, and earning little to no exp or loot for overcoming these challenges. There wasn't really any in world explanation for why even the most mundane thing was just as super as we were.
One of our players decided he wanted to play something new, so unbeknownst to the rest of us, he and the DM arranged for a suicide mission for him. My character joined him and died as well. I had no issue with it (other than the fact that his very beloved character, an orc barbarian war chief, went out to some thugs) as I was very much aware that death was a part of DnD. Plus, this would give me an opportunity to make a build that would allow me to tell the DM, no I am going to do the thing I want, and it WILL work. That's right, I made a lucky feat div wizard.
Now of course, I actually tried to flesh out the character. I gave him a great backstory that tied into the quest, the current party, and gave the DM a lot of room to work with. He was a "Time Wizard" (reskinned div and necromancy spells, as well as spells like haste/slow and hold person to be temporal manipulation). His whole build was "rewinding time" to give everyone a second chance to succeed. He was a deserter when the Royal Army tried to conscript all the wizards from the Royal Army. Since he had a lot of powerful people upset with him, he was really paranoid. I thought this tied well with all the divination spells he learned to help detect any threats. Phineas Howell.
The first encounter he had we of course found some monsters on the side of the road that were just as strong as the last boss we fought and so I thought there would be no better time to test out my "I decide you fail your saving throw" wizard. He forced a creature to fail a saving throw against hold person "trapping them in time". I then adressed my paladin:
"Todak, he's all yours."
Out of character: "You have advantage on attack rolls against him, and if you hit him its an auto-crit. Also, remember that doubles your smite dice. Also, remember your sword doubles smite damage."
Todak's player's lit up. With glee he smote that monster into next week. Over 100 damage from a single blow from a level 5 character.
There was silence at the table as it dawned on everyone what this meant. No more being troubled by thugs with 100+hp and AC of 20. We were now as much in control of the dice as the DM.
Or so we thought. Because, when you go to war against the god of your world, you are going to lose. The DM only responded by introducing even tougher enemies. And we encountered a cult of techno nerds that hated magic (think the equalists from Korra). When we first encountered them, Phineas told everyone to run as he would be useless in the anti-magic field the cultists' bracelets could generate. That's right. Entry-level grunts in this cult were given bracelets that could cast a level 7 spell as a bonus action, with the aoe magnified 10x (no seriously, the aoe was 100ft radius). This was standard issue for every grunt. Again, we were still level 5 at this point.
Phineas tells everyone to run and high tails it out of there. One or two others follow me but the rest decide to fight. They win and then when the DMPC catches up with us, he slams my character against the wall, holding him by his throat.
"Why did you flee you coward?! We could have died!"
"Well, I would be useless against them, and I told everyone to run. You all had the same opportunity as me. Torren ran as well. Why aren't you mad at him and attacking him as well?"
The DMPC gave some excuse about why it was okay for Torren to run and continued to badger me. I cast prestidigitation on his breath to make it a little more bearable. Nice and minty fresh.
Well from that point onward, the enemies' saving throws only got higher and higher. Even with some awful forced portent rolls and a maxed out INT stat, they still beat the saves sometimes.
Eventually, we encounter an actual psychopath assassin. Bumpus. Bumpus decides he is just going to join our party. We kinda get railroaded into letting him. Well, Phineas, paranoid as he was, was not having any of that. So I developed a plan. I was going to have our paladin kill Bumpus in his sleep.
I let the DM know ahead of time that this was my plan. I talked with the other players and they were all okay with it.
Phineas stays awake while everyone falls asleep and uses his familiar to make sure Bumpus is asleep. Then he uses his familiar to wake up the paladin and explain the plan by writing with his paw in the dirt. Via help from the familiar and portent rolls, Todak the paladin sneaks over to Bumpus and rolls to hit. Nat 20. Double smited. 150ish damage. Second attack. Another 75ish damage.
Bumpus the unholy psychopath assassin jumps up like there's not a scratch on him and just wails on poor Todak. After a two or so rounds we discover Bumpus' AC is something like 23 (higher than that of an ancient dragon) and his HP is something like 750 (higher than the tarrasque). He passes every saving throw and just demolishes our poor paladin. The DMPC sides with Bumpus as Bumpus continues to tear our party apart.
It turns out we were eventually supposed to fight Bumpus but only after finding the McGuffin that would allow us to fight on his level.
We flee but in the end as a result of physical, mental, and emotional damage, we lost three of our six characters. Some of the players decided their characters were just done with adventuring after that encounter, and one (Phineas) ended their own life after such a catastrophic loss.
I got rid of Phineas as I was tired of having magic users being consistently nerfed, and we lost our main damage dealer so I decided to make another min/max monstrosity. A human variant battle master fighter with archery fighting style, crossbow expert, sharpshooter, and alert. I rolled in front of everyone and was able to get his dex up to 20. I was ready to quick draw cowboy shoot this campaign into Oblivion.
I still hadn't learned my lesson. Of course, the more damage my character was capable of doing, the higher the HP of the monsters would be. The more accurate my attacks were, the higher the AC. The better I got at abusing dice manipulation mechanics, the harder the skill check DCs got. I caused a cold war arms race of the PCs versus the DM.
Eventually, after fighting being railroaded the DM messaged us to figure out what went wrong. Everyone was frustrated and it felt like we were against each other and the only way we could have fun was by beating the DM.
We had a heart to heart and we decided to put that campaign away for a while and play something else. I realized that min/max playing is only going to escalate threats and lead to conflict. The DM learned to be more collaborative and not railroad. We still play in the same group and we are loving our campaign, where I play a mute, totally nonoptimized water genasi land druid. The game has not been made any less fun for me limiting my character, and the new DM has been very collaborative.
I hope eventually we can return to the other campaign and finish it. It really was a great story and very cool world. Now that we are all a little more experienced, and a little wiser, I think it could wind up being beautiful.