I have always been the type of player that would try to come up with different themes for characters by pairing features from multiple spells or classes, and I had one that I was really really excited to try. It took me a lot of sessions to get my character to level 19 since that's when the build comes online, and I had fun getting there; but just when I used it my DM cheated me out of a single moment that would have been the culmination of 19 levels worth of struggle.
I'd first like to say that I've been playing this character in the Adventurers League, which meant that in order to get to level 19, I didn't have to play under just one DM; and true enough, I jumped from table to table on one-shot modules since my schedule wouldn't let me join a consistent campaign. That didn't make levelling up any easier, as it still took me a year to get to tier 4. I had a lot of fun getting there, running all around Faerun stopping one nefarious plot after another and meeting all sorts of adventurers, but I had my eyes set to when I can finally do the combo that my character has figured out early on but never had the power to perform.
The combo in question requires at least 2 levels in fighter to get action surge, 3 levels in rogue to become an assassin to get assassinate, and finally at least 14 levels in wizard under the school of evocation to get overchannel. I also required the spell steel wind strike (a spell from Xanathar's Guide to Everything) which a wizard can cast so I'm good with that already. The combo can only be done under the specific condition that it's a surprise round and that you outroll the enemies in initiative. I also took the spell sniper for good measure, it will be clear why later. It takes a lot for the combo to come to fruition but if all of those pre-requisites are met, one can deal 240 force damage to 5 enemies. Here's how it would go down, you cast steel wind strike on targetting 5 enemies, which on a surprised round will automatically be a critical hit when you hit due to the assassinate feature from assassin. Steel wind strike deals 6d10 force damage which meant that on a critical hit would deal 12d10 force damage. Seems low still, but when you use overchannel on the steel wind strike (which you can do since it's a 5th level wizard spell) it would deal the maximum damage of 120 force damage. Now I can hear you asking, that's only half of the promised 240 force damage, where's the rest of it? That my dear friends comes from using action surge and doing it again. Doing so however would deal 10d12 necrotic damage to the caster since you can only safely use overchannel once, so you can only imagine how hard it would be if that full combo did nothing to the enemies, which was the case on the first time I tried it when my cheater DM thought he'd just shrugged it off.
The adventure in questions was set in Chult. We were supposed to find a shard of the destroyed Soulmonger, the artifact Acererak was using to terrorized the world. The shard, despite being just a small part of the once great and terrible artifact, still contained a lot of power which a certain evil, red-themed, group of casters wanted to exploit for whatever evil reasons. The race to the shard was very one-sided with the red wizards getting a huge head start, while we're still busting our asses looking for clues on where it was exactly. When we eventually got a solid lead, we headed straight towards the Peak of Flames, a volcanic region found on the south of Chult. Not only were we fighting against the debilitating effects of the environment, the red wizards also hired a mercenary group that were tasked to ambush anyone heading towards their direction. Thankfully there were two strong casters on our party, and despite the exhaustion from travelling, we were able to come out victorious from the ambush. When we were close to what looked like a tomb, I told the party to go stealthily and I, along with my simulacrum of myself, volunteered to scout ahead while under the effects of the invisibility spell to increase our odds of hopefully catching them by surprise and increasing our chances of stopping whatever they were planning. The party agreed, but decided to stay not more than 50 feet from the entrance so that they can run in in-case something happened. I was supposed to contact them through our telepathic bonds if I see something so that we're all aware of what's happening, but upon entering the tomb-like structure within the volcano, I immediately saw the red wizard already in the process of absorbing the power from the soulmonger shard along with 5 Giant skeletons and a Death Knight all in a small area to protect him. When I asked the DM if the ritual to absorb the soulmonger shard was complete, he told me that it would take a couple of turns for it to finish. Not wanting to risk the red wizard to fully absorb the power from the artifact which might give him powers we were unaware of, I telepathically contacted my party to charge in post haste while I would engage them right then and there. I asked the DM if I could have a surprise round since they were unaware of both mine, and my simulacrum's presence because we both rolled above 27 on our stealth checks all the while being invisible. This was it, my character thought. What he and I have been waiting for for far too long. I told my DM that I would cast steel wind strike and would overchannel it to deal max damage, and since I'm an assassin, it's also a critical hit. I hit on all my attack rolls against their AC, which was okay for everyone on the Discord but when I told the DM that it would deal 120 force damage he told me to wait for a moment to check the spell. It was a routine DM thing to do so I didn't think much of it. When he spoke again he told me that he mispoke, and it turns out that they're all actually around 40-50 feet away from me and from each other, and that my spell wouldn't work since steel wind strike only has a range of 30 feet. I could practically feel the smug smile that he probably had from my end of the screen from the way he said it, but I pointed out that since I had the feat spell sniper all of my spells that would make an attack roll would have its ranged doubled, meaning my steel wind strike actually had a range of 60 feet, easily reaching all of my targets. Not wanting to be outdone, the DM then declared that he'll cast counterspell, which I pointed out he couldn't since surprised creatures can't take reactions. He asked for a few more minutes to himself, which I can only assume he spent looking for any reason to disallow my move. When he spoke up again, he told me to proceed with my turn; a gesture which I took as "everything checks out, you're good. To much of my happiness, I proceeded by declaring that I'll use action surge to do the same action again, and that I'd roll 10d12 necrotic damage while the enemies would take a total of 240 force damage. I ended up taking 84 necrotic damage from that but it was all worth it for the move that I have been waiting a year to try. When I asked how the enemies were doing, he told me that they're all still up, and were okay; even going as far as saying that they all had resistance against force damage which is why they were okay. I was weirded out by this since this wasn't the first time that I danced with red wizards who are mostly archmages of great renown. It didn't faze me though as I still had my simulacrum who can do everything that I can. He started with just the overchanneled steel wind strike since it isn't cheap to heal simulacrums, but when the DM said they were all still alive, I thought f*ck it, and declared that my simulacrum would action surge and do the same thing again. The move dealt 95 necrotic damage to my simulacrum, which was enough to kill him, and melt him down since he was already damaged from the ambush. At this point, I have already dished out 480 points of force damage to the same 5 creatures, and I thought that even with resistances a caster couldn't possible survive it. When I asked the DM how my foes were faring though, he told me that they were all fine. Dejected from this outcome since to me it was obvious than something fishy was going on, I simply declared that my turn was over having already used all the cards up my sleeves.
After my turn ended, all my partymates used their actions to dash inside the tomb, and when it was the Archmage's turn. He casted timestop which my partymate tried to counterspell, but when he did he was told that the red wizard used subtle spell on it which is why he can't counterspell it. Which was weird because, last time I checked archmages, which is the NPC equivalent of strong wizards, are the NPC equivalent of wizards and not sorcerers. During his 4 turns under timestop, he casted delayed fireball and a level 7 fireball at us. With only 30HP remaining, I was already unconcious from the delayed fireball, and I got 1 failed death saving throw from the fireball that came right after. I was devasted because of this, not only because my character is down, but because it felt as though the move that I was preparing to do for so long seemingly didn't even scratched any of my enemies. My teammates managed to heal me eventually and was able to down the archmage with their combined normal attacks and some spell attacks. I wasn't paying much attention at this point because I already felt so cheated.
After the game, I calculated the HP of the archmage and the death knights by adding all of the damage it was dealt. After doing so, I found that both of them somehow had around 540 HP if you take the DM's word that they did have resistance against force damage, and around 780HP if they didn't. I felt so cheated, and I felt like all the time I spent on this character was wasted. So much so that I didn't play D&D for a month; and when I continued playing, I was so sure that I won't ever play Jin, my assassin wizard anymore. That changed though when the call to adventure came, and I couldn't help but remember all the good times I've had playing Jin.
I have thought of multiple terms that i could call that DM, like power-tripping DM or just downright a bad DM, but a cheating DM felt most right when I was thinking about it because of the way he made me feel. I hope that other DMs both veterans and new ones won't do what he did to other people.