The time my players killed the main villain in one turn

For a bit of background, when I ran The Curse of Strahd for my group, one of the players wanted to play a wild magic sorcerer. Since I was fairly new to DMing, I didn’t really have a sense of when I should have him roll to see if he activated a wild magic surge, so I decide that whenever he cast a spell, I would roll a d10 and if it rolled equal to the spell level+1 or below, it would have a chance of activating a wild magic surge. At the same time I decided that if the wild magic surge made him cast a spell, it would also have a chance of causing a wild magic surge, following the same rules, and if a spell level was not specified in the chart, I decided that it would be cast at level 9, which guaranteed the activation of a surge.

With all this in mind, let’s get to the actual event. The party had battled their way through Strahd’s castle and made it to his crypt where they knew he was lying in wait. As they entered the room, weapons drawn, he sprung out of his coffin to attack them where I promptly rolled a critical fail with advantage, causing him to fall flat on his face. The sorcerer rolled the highest initiative and went first, casting a 6th level magic missile at Strahd. This caused a wild magic surge, which cascaded multiple times. By the end of it, Strahd was at 15 health. It then passed to Strahd’s turn, where, because the party’s paladin was wielding the Sunblade, Strahd took 20 unpreventable radiant damage. Since he was in direct sunlight from the blade, he was immediately destroyed.

At the time, I was gutted. This had been a months-long campaign, we had had several player deaths, and some great battles, and it ended in the most pathetic way possible. But years later, it is one of the moments that my players still talk about and bring up as their favorite memories. The fight was 100% a failure, there’s no denying that, but the sheer absurdity of how it ended makes them remember it in a good light.

If I ran the campaign again, or had another player use wild magic, would I follow the same ruleset I did then? Absolutely not. But, if given the chance, knowing what I know now, would I go back and change the original ruleset? Also no. That moment was unforgettable for our group, it is a core memory for us, and even though it was a failure for me, the players enjoyed their experience, and that’s what really matters, at the end of the day.