I'm a first time DM, so naturally I'm always unsure about how well I've balanced encounters. Often, I'll adjust the HP of the enemies behind the screen to make them more or less powerful so that my players feel challenged but not overwhelmed or overpowered. Normally, that works pretty well, but I have had a few unanticipated curve balls in the combats that I run.
So here's the story: The party is entering a dungeon I designed called the "Hall of Mirrors." They need to get an artifact and rescue a pixie that are both inside. Each room of the dungeon is a different mirror-themed puzzle, and I'm pretty proud of what I came up with for it. Oh, alright, I did also find several ideas online, but at least half of it I made up myself with no help.
Everything went really well up to the boss room.
The boss was a banshee, I think you all can see where this is going.
It wasn't just any ordinary banshee, of course, that wouldn't be interesting enough. I bumped up the health and added both legendary and lair actions to make it more dangerous, such as casting spells using the artifact the party is trying to recover. I also placed sharp mirrored spines of glass in various places around the room so that the banshee could bounce spells off of them. For the lair actions, it would vanish and reappear in three places throughout the room at the start of each round of combat and the players would have to figure out which one was real.
The party shattered some of the mirrors near the entrance, which really opened up the arena for them and made the end of the fight survivable. When they entered the room, they saw the banshee holding the artifact in one hand and the pixie in the other. They managed to use some stupidly high stealth rolls to replace the fairy with a piece of pickled flesh wrapped around a shard of warm obsidian (see the trinket table in the PHB for reference), and then the fight was on.
The fight went pretty well for a while, and I was proud of what I'd done. They were all challenged and each of the party got to contribute meaningfully to the fight. The sorcerer, our party's only full caster, was pouring his sorcery points into trying to nuke it with scorching ray and fire bolt. They're both good spells, but banshees are resistant to fire damage, so all that damage was getting halved.
I wanted to wait for a while before using the banshee's scream, since I'd heard of how it could be devastating to even high level teams, but I figured that it would be a really good way to shake things up if I knocked out a few of the party members. So, when the banshee was down to only 22 or so hp, I knew it was now or never.
THEY. ALL. FAILED. THEIR. SAVING. THROWS.
Except the sorcerer, who was nearly dead and almost out of spell slots and sorcery points. So while everyone else ragdolls, he's left alone with the boss.
He backs out of the room shooting off spells at the banshee to lure it away from the party. It chases after him and, due to some poor rolls on my part, he doesn't just die right away. Meanwhile, the ranger and rogue succeed on their death saving throws on the same turn that the banshee downs the sorcerer. They distributed potions to the barbarian and monk and they all set up to attack once the banshee came back through the doorway.
Instead of waiting, however, the rogue took off down the hall and into the room where the banshee was standing over the unconscious sorcerer. He barely made it close enough to cast burning hands and finally finish it off.
Not going to lie, I was legitimately worried that I was about to kill them all, and this was the first session my girlfriend (now wife) was playing with us, so I was not wanting to just kill the character that we'd helped her make.
"Hi, great character you spent hours on! And DEAD!"
But anyway, back to the story, it was almost time to wrap up the session, so the party gave the unconscious pixie they had rescued a health potion and the sorcerer sealed up the boss room using mold earth so they could take a long rest. I know they say never to let the party take a long rest in a dungeon, but these guys earned it.
Thumbnail image credit: Licorne37