YMMV / Angband

• Demonic Spiders: Loads and loads of them - in fact, a key part of Angband strategy is just not to get into fights with creatures like these.
  • Hummerhorns (giant wasps) have a horrifying combination of two abilities; a bite that causes confusion, and the ability to self-replicate. If you have just one of these on your hands and you don't deal with it immediately, you're in for a world of trouble - its confusing bite will hinder you from acting while it happily replicates around you, surrounds you, and bites you to death. Even a high level player can be taken down by these if they haven't taken precautions.
  • Zephyr hounds come in huge packs and have various breath weapons, which they won't hesitate to blast you with.
  • Quylthulgs are immobile piles of rotting flesh with various summoning and teleportation abilities.
  • Drolems are the reason you never go below 2,000 feet without poison resistance. They're dragon golems with powerful poisonous bites and breath weapons - and being golems, they're immune to many things that biological dragons wouldn't be.
  • Ancient Multi-Hued Dragons: Multi hued dragons resist all basic elements making them hard to kill and they can breathe all of them too - if you don't have all resistances, this can be a One-Hit Kill.
  • Nether Worm Masses are the reason you don't go below level 20 without "see invisible". They are invisible, bleed explosively, can tunnel through walls and have an experience draining attack. Without the ability to see invisible, the first warning you'll have is when you are hit by something and start losing experience points and you can quickly and easily be surrounded by explosively breeding experience draining worm masses that you can't see and even retreating into a corridor won't save you as they tunnel through the walls to surround you.
  • "It moves very quickly"
• Game-Breaker: Like in most roguelikes, being fast. The most dangerous monsters are those that can hit you 5 times each time you move. The game is at least aware of this; the manual notes that speed is one of the most highly sought attributes to improve (and thus, speed enhancers are very hard to come by).
• Goddamned Bats: Oozes, icky things, mushroom patches, and worst of all, worm masses.
• Goddamned Boss: Smeagol is a low-level boss who isn't particularly dangerous, but is quite annoying; he has the toughness of a boss combined with the ability to steal your gold and teleport away. At the time you first encounter him, it's unlikely you'll be able to kill him, so your only option is to run or lose your gold. He's also invisible, so the first you might know about his presence is when he steals some gold and has an absurd number of hitpoints for his level making him impossible to kill for a low level character before he steals gold and teleports away.
  • Wormtongue to a lesser extent, as he is actually threatening.
  • Any boss that summons. Especially when those summons can (and do) summon themselves - this can lead to a exponential summoning spiral which is impossible to keep up with.