Avernum/YMMV

Complete Monster (or Magnificent Bastard): Gladwell seems to have done more damage to Avernum through sabotage than any organization, demon or dragon. And even though he earns a death at the hands of the player in every game he's appeared he canonically won't stay dead until perhaps the end.
Demonic Spiders:
- Several things are there to ruin your day, including slimes that split into two whenever you hit them. Then there's the Doomguard, which is a boss monster that splits into two whenever you hit it. On normal difficulty, it's just a very long fight that saps every resource a party has. On higher difficulties, Save Scumming is highly recommended.
- Chitrachs deserve special mention, not because they're all that tough compared to Doomguards, but because the Eastern Gallery in the fourth game was made into a Scrappy Level purely by their presence. (To be more specific, more than two hundred of them were present.) And chitraches tend to be near null bugs, a creature who can render mages/priests useless with the terrible act of... being close to them. Yeah.
Goddamned Bats: The gremlins. They're not tough, not very fast... but their only ranged attack is a confusion ray. And they also run away while everyone is confused. To make it even more frustrating, in the original trilogy, they attack twice a round, almost always come in large groups, have no variation in their strategy, and confusion can make a character skip their turn, so 90% of the fight is just waiting for them to finish their attack animations.
Good Bad Bugs: The first three games use a reverse form of Fight Woosh to put your party back into formation after a battle. If you're already in formation, your group may be moved forward a few tiles, potentially bypassing an event scripted to occur when you step on a tile.
It's The Same Now It Sucks: Avernum 4's plot falls into this category for some.
That One Disadvantage: Two of them, actually--Cursed at Birth and Completely Inept.
That One Level: Avernum 4's Eastern Gallery manages this entirely through Demonic Spiders, giant insects known as chitrachs. They're fairly hard to hit, they have high HP and damage resistances, and every time you perform a melee attack on them they have a chance of blocking and damaging you. Oh, and there are 227 of them out for your tender flesh. Enjoy!
- The Spiral Pit in 1/Escape From the Pit. It lulls you into a false sense of security by siccing low to mid level undead at you, but halfway through the dungeon, the lights go out, severely limiting your vision and maneuverability, you have to fight through half a dozen Stalker Wights, and the last fight of the dungeon is against Ghosts that can do over 100 damage in a single attack.
- The Slith Fortifications from the same game(s). On paper, it doesn't look so bad, just a bunch of Slith flingers, mages, shaman, warriors, their pet lizards, and their chieftain. The problem? There are a ton of them, moreso than most of the other Slith dungeons you've been to thus far. Not only that, but the map itself is ridiculously small and cramped, meaning that it is very, very easy to aggro literally the entire fort if you don't know what you're doing.
They Changed It, Now It Sucks: Many fans felt this about the engine switch the series underwent for Avernum IV through VI. Also a case of They Copied It, So It Sucks--the later games are nicknamed "Averforge" for their similarities to Geneforge.