I was thinking back to my most recent use of these monsters. A fun little exercise.
Black Pudding: A guardian of the vault of Mazhug, the Last King of the Orcs in a dungeon. It first appears as inky darkness when the vault door opens. Once touched, it awakens to attack. Under the floor of the vault is Mazhug's hoard. This got the better of one of the PCs in at least two groups I ran this for.
Gelatinous Cube: A wandering threat in the upper dungeon in the Get Bargle adventure I'm running now. The PCs bullied it with telekinetic shoves and rays of frost. Sad!
Carrion Crawler: Same dungeon as the gelatinous cube. Not much of a threat.
Mind Flayer: The main villain, Doctor Lobotomy, in a dungeon I run for one-shots called Startertown Underground, a post-apocalyptic sort of dungeon setting. He's been confronted by only one party so far who were brave enough to enter the Sphincter Beyond Comprehension, a fleshy portal to his lair. Just one PC escaped and he was transmogrified by the portal. I later used Doctor Lobotomy as a supervillain in my Street-Level Heroes campaign.
Intellect Devourer: I actually can't remember the last time I used one of these. I don't really care for them for no good reason. I have used them, just can't recall in what context. Maybe in my last Eberron campaign.
Roper: Last time I had one in a game was when I ran the 5e version of Forge of Fury. It's a pretty good monster to reskin for anything you want to grapple and reel though. I've done that several times. I also featured a roper in my short-form scenario,
Rope Tricked.
Umber Hulk: This was a minor villain in my Street-Level Heroes game. Its name was Digdug. On his Right three fingers he had a tattoo of D - I - G. On his left three fingers, he had a tatoo of D - I - G. Luckily for the tattoo artist, Digdug was illiterate. He killed one of the PCs, Dr. Squid, by collapsing a cave ceiling on him.
Mimic: I used one in a Lovecraftian dungeon. It disguised itself as a sarcophagus sitting atop the pressure plate for an acid-spewing trap. So PCs would mess with the sarcophagus, get stuck, and get acid spewed on them. Good times. I killed a PC in a different dungeon with a mimic next to an electrified floor plate. Haha, zap, one more monk dead before he could get stunning strike.
Rust Monster: Love these because nothing raises the stakes like the threat of equipment degradation. They are wandering monsters in the current Bargle dungeon I'm running and are the very first encounter in Startertown Underground that I mentioned above. That adventure is really about finding a lot of junk to use and improvise with, so taking out equipment plays into the theme.
Displacer Beast: As mentioned upthread, these are on the Forest Encounter chart from XGtE which I used in the wilderness trek to and from the dungeon. They came up once and like 4 of them, so those Tier 1 PCs were very lucky to escape.
Beholder: A villain in my Street-Level Heroes campaign named Perceptus who lived in a hollowed out tower in a swamp.
Marilith: Serpentina, the captain of a
Ship of Chaos in my current Planescape adventure, God Heist. This Friday the PCs plan to try to steal the ship so they can get to their goal quicker, no doubt meaning they will fight this menace and her crew of glabrezu. This battle is taking place in Graz'zt's realm in the Abyss, above Zrintor the Viper Forest. Pretty cool backdrop.
Tarrasque: I've never actually run a game with a Tarrasque. I have a half-designed scenario involving one destroying a city and the PCs basically need to mitigate damage as the goal and eventually figure out how to deal with it by some means other than flying and acid splash. But I did recently play in an adventure where we were swallowed by a tarrasque and had to fight our way to the exit. Yes,
that exit.
