There are many annoying cards in the encounter deck in any given scenario, but some can actually end your run very quickly, either by wiping out the team, or by advancing Doom to the point that you run out of time.
- From the core game:
- The "Ancient Evils" treachery has just one effect — it places one doom on the agenda, effectively leaving the investigators with three fewer actions each to finish the scenario. Additionally, it is one of the few doom-placing effects that can advance the agenda right on the spot as opposed to waiting until the start of the mythos phase. Cards that follow the "Ancient Evils" template in other campaigns (or otherwise fulfill a similar function) are generally just as nasty as the original, and just as loathed.
- The "Wizard of the Order" enemy from the "Dark Cult" set is like the Acolyte, but instead of entering play with 1 doom, it generates 1 doom at the end of the mythos phase of every turn. It spawns at an empty location, and if left alone, it can cost the investigators the game.
- In The Dunwich Legacy campaign, the "Beyond the Veil" treachery (from the recurring "Sorcery" set) is put into play in an investigator's threat area, but does nothing... until their deck runs out of cards, at which point it deals them ten points of damage - enough to One-Hit KO most investigators, as even the tougher ones only have 9 health at most. And there are plenty of effects in The Dunwich Legacy campaign that discard cards from investigators' decks directly, with one such card also being in the "Sorcery" set.
- The Path to Carcosa campaign:
- The "Seeker of Carcosa" enemy (from the "Echoes of the Past" scenario) quickly sucks out clues in its location, and once they run out (which happens quickly, as there are the enemies who do the same), starts accumulating Doom. With three of these fellas being in the deck, they can end the scenario very quickly.
- The "Dismal Curse" treachery from the "Dim Carcosa" scenario starts with a test difficulty of 3 and deals 2 horror on a fail, but gets +2 to both for investigators who've suffered too much horror, making it both hard to avoid and hitting like a truck.
- "The Final Act" treachery from the "Dim Carcosa" scenario works exactly like "Ancient Evils", but has doubled effect (taking away two full turns in one shot), and immediately "surges" into another encounter card.
- The Forgotten Age campaign:
- "Ill Omen" treachery (from "Forgotten Ruins" set) is like "Ancient Evils", but worse. It puts the Doom tokens on locations, where it stays even if Agenda advances, so the more Agendas scenario has, the more time in total it chops each time it hits. It also has "Peril" keyword, preventing the other players from assisting in dispatching it. And as a tip on the cake, there are many locations with additional penalties if there's Doom in them.
- "Ancestral Fear" treachery from "Forgotten Ruins" set forces the players into a Sadistic Choice: either it adds Doom to a location (similarly to "Ill Omen" from same set), permanently taking away some time, or rises Vengeance (albeit this at least removes it from the game), which would hurt the players even after scenario is finished. It has both "peril" and "surge" keywords, meaning that it's hard to cancel and it would be immediately followed by another encounter card.
- "Conspiracy of Blood" treachery from "The Threads of Fate" scenario lowers Doom threshold of the current Agenda (and can stuck up to two times), which can screw you over if something else rise Doom. It can be disabled after triggering, but it requires going out of you way, and imposes Willpower test which makes things worse if failed.
- "Window to Another Time" treachery from "The Boundaries Beyond" scenario is another Sadistic Choice card; either it hits like "Ancient Evils" and takes away your precious time, or it shuffles "Ancient" location back into exploration deck, possibly undoing several turns of your hard work, without any guarantee that you would be able to catch up. As may be expected, it also has "peril", so no outside help.
- "Lost Humanity" treachery from "The City of Archives" forces you to pass a hard Willpower test, and removes your cards from the game if you fail, with effect scaling with your performance. It's very nasty effect in the best of times, but here, you're stuck with low stats, without many of your powerful cards, scenario's goal is to gather cards in hand, and it can drive you insane if it reduces you below certain amount of cards. Additionally, reducing your deck's size means looping through it more often, which, in turn, means extra horror.
- The Circle Undone campaign:
- Piper of Azathoth enemy from "Agents of Azathoth" set is tough like a tank, hits even harder, relentlessly pursuits investigators and can't even stay dead once slain, despite supposedly providing victory points, as the other cards which comes with its set can revive it. It also has "Elite" trait on top of it, despite not being an actual boss, which protects it from most of the best tools at players' disposal.
- Witness of Chaos enemy from "The Clutches of Chaos" scenario puts breaches (read: Doom) in every location she enters. She spawns in location with fewest breaches, potentially far away from you, and then starts running towards you, due to also being a "hunter". The only adequate response is meet her head on... where she reveals high Fight and health values. Fortunately, she awards victory points, which means that once slain, she stays dead.
- Mindless Dancer enemy from "Before the Black Throne" scenario is like Piper of Azathoth's little brother: only slightly less health, even more Fight, and targets your health instead of your sanity. Not only it hits like a truck, it runs like one, too, being able to pass through "empty spaces" and even receiving extra moves when doing so. It isn't Elite, but there are three of them in the deck; they also don't award Victory points, so even if killed, they can return later.
- "The End is Night!" treachery from "Before the Black Throne" scenario imposes (increasingly higher) Willpower test, and on a failure, feeds all Doom on Cultists in play directly to Azathoth (or 1 Doom if there's none). If Azathoth ever accumulates at least 10 points of Doom, it's an immediate Non-Standard Game Over, regardless of current Agenda.
- The Dream Eaters campaign:
- "Myriad Forms" treachery from the "Where the Gods Dwell" scenario. Once drawn, it forces to reveal all copies of Nyarlathothep in your hand, and then shuffle them back into encounter deck (but not before they attack you). What makes it truly dangerous, and not just annoying, like most time-wasters, is just how much time and efforts gets thrown out of the window, and how hard it's to come back from it; just one such card (and there are two of them in the deck) can irreversibly ruin your run.
- Web-Spinner enemy from the "Weaver of the Cosmos" scenario. It's an elusive spider which hides in the distant empty corner and, unless found and smashed in time, puts Doom on its location with every turn (which is permanent, persisting between Agendas), directly contributing to your defeat. As they have "aloof" keyword, by the time you move to them and engage, you would already spend two out of default three actions, and if you fail to kill or exhaust them (by evading) with the last one remaining, they would still do their black deed.
- In The Innsmouth Conspiracy campaign, a Pursuing Motorcar enemy from "A Horror in High Gear" scenario, which can attack everyone in a car, doing 2 points of damage; it has above average Fight and health, meaning that it's likely to attack more than once before the players deals with it. And if it catches someone outside of the car, its damage doubles (even bosses rarely causes more than 3 damage in one attack).
- Edge of the Earth campaign:
- "Polar Vortex" treachery from "Deadly Wether" set sticks to a location, and, if they end a turn there, deals direct damage to not just investigator, but all assets under their control, partners included. While investigator often can survive it, same can't be said about many partners — and those who can, would keep their damage between scenarios. What makes it much more dangerous than its twin sister "Nebulous Miasma" (from "Miasma" set; targets sanity instead) is that it shows up in scenarios with way fewer options to avoid it.
- "Ice Shaft" treachery from "Dangers of Antarctica" set at first looks like yet another "fail the test, get 2 damage" card, but what makes it more lethal is campaign-exclusive frost tokens, which not only can make you fail this test automatically if you draw 2 of them, but this card also deals you extra damage per frost token, resulting in up to 4 damage from just one card (and there are three copies of it). It has a twin sister, "Dark Aurora" from the "Silence and Mystery" set, which causes all the same troubles, but targets sanity instead of health.
- The Scarlet Keys campaign:
- "Substance Dissimilation" treachery (from "Outsiders" set) causes you 1 damage and 1 horror for each card in your hand and play area which have a set-aside "hollow" copy. Given all the ways the game can create "hollows" (including other cards from same set), the damage can quickly accumulate to lethal levels (3/3 damage/horror is a boss-grade, and it can go beyond that). If you don't have any "hollows", it instead "hollows" one of your cards.
- Ancient Raider from "Dead Heat" scenario, whenever it attacks an investigator, kills a civilian in their location. While attack itself isn't much of a threat (they only do 1 damage), civilian dying is, as it can severely screw investigators later and cause them mental traumas. They have "Retaliate" keyword, so if you fail while attacking them, they kill someone when fighting back, too.
- Sinister Aspirant enemy from "Dealings in the Dark" scenario is like Wizard of the Order, except there're three of them in the deck. Just like the Wizard, Aspirant keeps accumulating Doom if leaved alone, albeit it has different statline. Good news: they don't spawn in empty locations. Bad news: for much of this scenario, they (along with aforementioned Wizard, and the Acolytes) would be "Concealed", so you have to expose them before killing.
