Pray that the Dice Gods have mercy... because the Demonic Spiders most certainly will not.
- Dungeons & Dragons has had several examples over the years.
- Spider Swarms and Leech Swarms in 3.5 are terrifying for low-end characters: They automatically hit any enemies they're touching for non-trivial damage, are resistant or immune to most normal weapon damage, stack poison on top of that and make it so that a character can't actually run away
because he's covered with spiders that are biting him to death. Warlocks get the option to call such swarms at will, right at the first level if they want. - In 4th edition, Needlefang Drake swarms are Demonic Spiders to low-level characters. They have the ability to knock down characters, and then deal extra damage to characters who are knocked down in addition to the standard swarm ability of attacking all adjacent enemies at the beginning of their turn. Additionally, they take half damage from most attacks, and it is likely only one character in any given party has a reusable attack which deals decent damage to them at the very low levels you fight them at. One is dangerous, but manageable; two is likely to result in player death. Three or more are very likely to kill the entire party. While trivial later on due to being low-level enemies, to level 1-3 characters they're nightmarishly deadly.
- Skirmishers in general, and flying ones in particular, tend to be this to PCs of all levels. Their speed means it's difficult to focus fire on them, and they often have the ability to ignore or slip out of any status effects the PCs throw at them to try and lock them down.
- The Bebilith and Retriever are actual Demonic Spiders, though the level 18 Solo Bebilith doesn't really qualify for the trope. The level 27 Retriever fits the quite nicely for Epic-tier PCs, and even comes in an upgraded boss-fight package in the form of the 30 Solo Retriever Holocaust.note
- Monstrous Crabs in 3rd Edition, otherwise known as "That Damn Crab." The challenge rating implies that a 3rd level party can reasonably defeat one. Despite this, they are large and have long reach accordingly that dissuades melee combat, lots of HP, move fast, hit hard, and are hard to damage, and have a ridiculously high grapple modifier. Your average 3rd level party seldom has anything that can debuff one since its immunities let it No-Sell most of what the party wizard/cleric/druid can do. A typical encounter with one usually has it charging the nearest character for both claw and constrict damage, often one-shotting them. Next round, it tosses the downed character aside and repeats the first round, and subsequent rounds have it doing the exact same thing until the survivors flee or hide or it kills them all. There is a reason these things are not popular to fight against. One CR calculator found that the Monstrous Crab was closer to CR 5 or 6. Gratefully, the iconic "That Damn Crab" was only ever seen in a web enhancement, and when a Monstrous Crab showed up again in the book Stormwrack, it was significantly nerfed, with about half the HP and its damage cut by a good margin.
- In older versions (i.e. pre-4th edition) powerful undead were a nightmare. They were often immune to non-magical weapons, you could not get close to them without risking them Level Draining you to death, and if you died to them, you could often count on joining them, making the battle that much harder for whoever was left. A powerful cleric and his power to turn and destroy undead was utterly essential to survival against groups of them.
- Going all the way to Basic D&D (the 1977 release which mirrored Advanced D&D, 1st edition), all undead, even the lowly skeleton, were this to a 1st level party - because they were immune to morale checks. When almost any PC can die to a single blow, the most reliable way of surviving an encounter was to get the monsters to run away.
- Incorporeal undead in 3rd edition, as they are immune to non-magical attacks and have a 50% chance to avoid most magical attacks. Special mention goes to the Allip: its Challenge Rating is 3, which means a level 3 party can be expected to run into one— probably before most of the party has magic weapons. With its Turn Undead Resistance, a level 3 Cleric is unlikely to be able to hurt it. Its attacks deal ability drain, which can kill much faster than normal HP damage and which a level 3 party is unable to heal, and each hit increases the Allip's current HP. Finally, its mere presence has a good chance of mesmerizing half the party for about half a minute. If that doesn't sound much, by DnD rules, that equals 5 rounds.
- Rust Monsters. They don't do much damage, and are easy enough to kill, but will go directly for whomever has the most metal - usually your plate wearer - and try to dissolve it all with a touch attack, something that's usually quite effective against plate wearers. And hitting them with something metal will cause that to dissolve too. Even magical items can be effected. This is a game where losing your precious +4 Armor is a bigger problem than dying — if your party survives the encounter, you can be brought back to life for 5,000 gp, but a +2 weapon already costs 8K or more. Furthermore, replacing those items will take time, which is something you don't have in a dungeon crawl. Add all that up, and a Rust Monster has the potential to cripple a party so badly that they can't fight back.
- The tentacled Cthulhumanoid Mindflayers possess inherent psionic prowess, a hard to resist instant-stun at-will Mind Blast with a huge radius, and their ability to eat your goddamned brain is combined with ludicrous spell resistance and saving throws. Furthermore, as D&D goes on, they get more and more variations and options because they're such an iconic monster. How about fighting mindflayers with sorcerer/psion levels that can dispel your freedom of movement, or mindflayer liches with damage reduction, or maybe the batshit insane spiked-chain wielding monsters that are the Mindflayers of Thoon?
- In early editions (before 3.0), many level-draining Undead with Energy Drain could simply take levels away. It was much crueler than Negative Levels, either a temporary debuff which could become permanent if not cured, or saved against, or a permanent debuff that can be cured, in the newer editions. In early editions, Energy Drain meant you could start an encounter as a 10th level character, get hit once by a vampire's attack, and now you are an 8th level fighter. Permanently. The vampire is still around, and now you are less likely to defeat it quickly and thus more likely to be hit yet again. Three unlucky rolls later, your character might be 2nd level. Go back to the rulebook and take a look at how many HP and XP were just deleted. The only way to recover was to survive the encounter and then get back to adventuring so you can re-earn all those XP and levels. Negative Levels were added because Energy Drain was so disheartening, a player likely would rather simply have a beloved character killed, which could almost always be undone with a loss of one level at most.
- In AD&D, ghosts are able to make even the most potent characters cry. Their first attack is a possession effect. If the ghost takes control of you, there is no reason for it not to use your body to do as much harm as it can, then when at risk of expulsion, to impale itself on your weapon. You'd be dead, and the ghost would be fine. If you survived that, the sight of a ghost might make you panic and age 10 years on sight. Every hit from a ghost ages you an average of 25 years, quickly leaving you an ancient husk. Magic does not harm them unless you are also ethereal, and they are immune to non-magical, non-silver weapons.
- Spider Swarms and Leech Swarms in 3.5 are terrifying for low-end characters: They automatically hit any enemies they're touching for non-trivial damage, are resistant or immune to most normal weapon damage, stack poison on top of that and make it so that a character can't actually run away
- In the 1978 Avalon Hill board game Magic Realm the giant bats are among the most dangerous of all monsters. They are too fast to run away from or hit reliably, kill unarmored characters instantly, and wound to death those with heavy armor.
- Magic: The Gathering has a few:
- Ornithopter. Useless on its own, but with an Unholy Strength or Rancor, we're talking about two damage every turn. Played on the first turn. Or worse,Ensoul Artifact, for a 5/5 attacking as early as turn two.
- Squee, Goblin Nabob. Not to attack, mind. Squee works differently. Squee can be discarded or played and sacrificed. Then he returns to your hand. And he's a goblin, so all those "Sacrifice a goblin..." cards qualify.
- Any Rebel. Yes, all of them. Their abilities allow them to fetch more Rebels, which allow them to fetch more Rebels, which allow them to fetch more Rebels...
- Rootwater Thief: Flying creatures can only be blocked by other flying creatures, and if he connects, he can remove any card from a player's deck, crippling many deckbuilds.
- Voice of All. It's a small creature with flying, but when it comes into play, it gains protection from one color. Not just a specific color, however, but any color. Therefore, Voice of All could simply continue smashing in the face of any monocolored deck with impunity, or worse, create a near impenetrable defense.
- Disciple of the Vault is a dirt-cheap common creature that makes the opponent lose life whenever its controller loses an artifact. Since the life loss was unpreventable, there were a ton of cheap artifacts, the creature itself was hard to kill, and its power stacked with multiple Disciples, Disciple of the Vault was one of the most aggravating creatures in the infamous Affinity decks due to how difficult it was to stop.
- Bloodbraid Elf is a cheap creature with 3 power, haste, and the "Cascade" ability, which gives whoever plays it another cheap spell for free. So it hits hard the turn it hits play, and will probably hit hard again on that same turn. The deck archetype it was most often used in— Jund— used the Elf to dig up a Sprouting Thrinax, Blightning, or Maelstrom Pulse for free, effectively netting you two creatures, a shot to your opponent's dome and two cards out of his/her hand, or a quick switch AND three damage for the price of one. In any other deck, that's useful. In Jund, it's deadly. This got so bad that Bloodbraid Elf was banned in Modern to prevent Jund decks from dominating the format.
- Scars of Mirrodin has Plague Stinger, a small second-turn creature with infect — an ability that allows you to win by piling 10 or more poison counters on an enemy. Normally, this could be solved by blocking the creature; however, Plague Stinger has flying, making it especially hard to stop. This means every turn, unless you get some removal or a blocker with flying or reach, the Stinger is going to end up biting you to death, piece by piece. Throw in a pump spell or some Proliferate, and Plague Stinger goes from moderately dangerous to a game-ending threat that has to be answered.
- New Phyrexia one-ups the Plague Stinger with the Blighted Agent, a blue card with the same converted mana cost and Infect ability. It can't fly, though. But it doesn't need to: it can't be blocked by anything.
- In a similar vein to both, there's also Inkmoth Nexus. You can play it on the first turn and by the second turn, it can attack (making it one turn faster than Stinger). It's stats are identical to Stinger when it's a creature. New Phyrexian managed to up this threat by introducing Mutagenic Growth, essentially letting you whack at the opponent for up to 9 poison counters on the second turn. Bear in mind that you only need 10 to kill someone. This thing is also uncounterable, as it is a land coming into play. This means you have to use a kill spell on it while it's a creature or one of the expensive (mana-wise) land destruction cards, making it even harder to get rid of.
- In the Commander variant, several commanders have a reputation of "kill on sight," sometimes even extending to killing the player who controls them:
- Kaalia of the Vast will swarm the battlefield with free angels, demons, and dragons.
- The Mimeoplasm is usually big enough to kill a player regardless of their life total, and often comes with some annoying, undercosted abilities, to boot.
- Some commanders are known for comboing with a single other card to instantly win the game, like Mikaeus, the Unhallowed; Riku of Two Reflections; and Teferi, Mage of Zhalfir.
- And that's just commonly played casual decks. Tier 1 decks often rely less on their commanders, but feature regular creatures like Hermit Druid, which, through a convoluted series of actions, can win by turn 3.
- Kaalia of the Vast
- Ornithopter
- Pathfinder has many:
- Will-o'-the-Wisps are death machines to low-level parties. They have natural invisibility, flight, very high AC, immunity to magic, and touch attacks. You'll need to make special preparations — such as having a load of Magic Missiles (one of the two spells which are excepted from its immunities).
- Swarms can be this to low-level players, since some are immune to basically any type of damage that isn't an Area of Effect spell. Otherwise they're typically Goddamned Bats.
- They often get worse at higher levels, when their HP is too high for commonly-available AoE items, such as firebombs, to have any effect. They're weak to Area of Effect spells, but because these spells are practically useless against anything else, most wizards don't have any prepared.
- The swarm that can most quickly Total Party Kill an appropriate level party? CR 3 Wasp Swarms, meant to challenge neophyte adventurers. They administer poison, fly faster than you can run, and are immune to weapon damage. Couple this with more HP than a normal 3rd level party can do in area of effect damage, a chance that a character stung by the wasps does nothing but feel sickened and take damage each round, and the ability to chase down any fleeing characters, and the wasps will likely kill any 3rd level party which doesn't have a fair amount of luck coupled with an unusual (and suboptimal for most situations) focus on area of effect attacks.
- Almost any monster with the Grab special ability, which allows mobs to grapple you. Bonus points if they have the Constrict special ability, which means they automatically deal extra damage just for grappling. The worst are usually plant-type mobs, since they have a lot of immunities, high CMB, and are virtually impossible to detect in their natural environment until they attack. Examples include Shambling Mounds.
- Iron Gods introduced robots. They almost universally have high hardness values for their CR, and hardness is even more of a pain to deal with than regular damage reduction. The strongest ones also have force fields which nullify critical hits and sneak attacks as well.
