Decay and Filth

My take on this set: This set is almost exclusively carried by Corrosion, the other two cards are little more than filler for the encounter deck.
There’s not a whole lot that can be said about this encounter set. The enemy is just a random minor enemy in practice. Ooze and Filth is almost a dead draw. And Corrosion is… well, it’s one of the best asset destruction cards in any encounter set. It’s one of those rare cards that you should keep in mind when it’s added to a scenario because it can influence which cards you want to play (or in this case, keep in your hand).

What it does: Roach Swarm is a minor enemy with 2 stamina, so it goes down in one hit from any armed investigator. Its gimmick is having a fight value that scales with the shroud value of its location.

My take: This is not an enemy to worry about, it’s basically filler that is only one step above Swarm of Rats. The fight value can randomly be large, but if that is really a problem, the investigator engaged with the Roaches can always move to a connecting location. The enemy only deals a single point of damage, so that would be something to consider.

Threat level: Low.

Dealing with it: It’s a very basic enemy that can only pose a minor problem in edge cases where the shroud value puts it out of reach for the enemy handlers. But even then moving to a connecting location can defuse this enemy fairly well.

What it does: Corrosion makes the player discard Item assets from play or from their hand with a total printed resource cost of at least the current shroud value. Should no cards be discarded by this effect, Corrosion surges.

My take: UGH. This card is one of the most memorable from all of Carcosa for me because it often ends up costing me a whole lot. There is usually not much choice behind what to discard, as the requirement for the asset to be Item traited narrows down the options. And if you are not currently standing at a low shroud location, you can’t even feed some Fine Clothes, Flashlight or Scroll of Secrets into it.
This consistently hits harder than many other comparable cards (like Crypt Chill), discarding multiple cards at a time.

Threat level: High. If things go really sideways, this does not only discard cards from play but some from your hand as well.

Dealing with it: The important part here is understanding that the limitation to Item assets usually does not work in the players favor because it takes away their choice in what to discard. However, Corrosion also allows discarding cards from the hand to satisfy its demands and that is often preferrable because the player did not yet invest actions and resources into them. So if Corrosion is in the encounter deck, consider holding back an expensive asset in your hand (like a .45 Auto or a Fingerprint Kit) as insurance.

What it does: Ooze and Filth attaches to the agenda deck and is discarded at the end of the round. While active, each location’s shroud value is increased by one.

My take: A minor inconvenience. Upping the shroud for one turn usually doesn’t do a whole lot. There is some feedback with the other two cards from this set, but for that to matter you’d have to draw them in the same round. And in the right order. It could be a bigger problem if drawn in multiples. But with only two of them in the deck that’s not likely either.
I think this card could have Surge and i still wouldn’t think too much of it.

Threat level: Very Low. Can easily be worked around.

Dealing with it: Barely worth dealing with in the first place. Raising the difficulty of investigation tests by 1 might cause a test to fail, but even if it does that just means that the card cost someone an action.

Return to Carcosa: Decaying Reality

My take on this replacement set: An improvement, even if i think that Corrosion is a much better card than Fragile Thought is. Decaying Reality takes the two irrelevant cards from Decay and Filth and combines them into one. The now open slot is filled by a treachery that is generic but fine. This set is now no longer carried by just one of its card, this replacement offers a much better spread of power among its pieces. As a result, the whole set is more even. An good replacement.
Personally, i do use Corrosion instead of Fragile Thoughts in this set sometimes. Or one of both. As much as i hate it when i have to discard my stuff to Corrosion, i can not deny that it is a very neat card that has interesting implications on how to play with it in the deck.

What it does: Like the Roach Swarm it replaces, the Maggot Swarm is a minor enemy that should pose little issue to defeat. It increases the shroud value of its location by 2 and will move via the Hunter keyword while there are no clues on its location.

My take: Instead of the global +1 shroud from Ooze and Filth, this card will give +2 to just one location. This is a more significant upgrade for sure and since this one sticks around and isn’t discarded at the end of the round, someone will at least need to adress the creature. Of course, this isn’t terribly difficult and it’s spawning directly on top of an investigator who can then kill it.

Threat level: Low to Mid. Maggot Swarm combines the effects of Roach Swarm and Ooze and Filth into one card, which makes it matter a lot more than the original cards. Still, it’s just a 2 health enemy.

Dealing with it: The sooner you kill it, the less chance there is for the shroud increase to matter. Luckily, this enemy isn’t tough at all.

What it does: Fragile Thoughts replaces Corrosion in this set. Like its predecessor, it causes the player to discard cards with a combined printed cost at least equal to the shroud value of the location. Instead of Item assets in play or in hand, this one goes after event cards in hand, though.

My take: Often, this card lacks the impact that Corrosion has. The reason for this is twofold: One, events are a broader category of cards than item assets, meaning the player often has more choice in what to discard. Two, this only can go after cards in hand. Corrosion’s worst case is having to throw away assets in play that already were payed for in actions and resources, something that Fragile Thoughts will never ask for.
However, Fragile Thought is more likely to hit multiple events, sometimes even stripping all of them from a player’s hand. The reason for this is the comparatively low cost of events. Many only cost between 0 and 2 resources, so someone at a 4 shroud location could easily have to discard all of their events. To clarify, if the shroud value can’t be matched by the events costing 1+ resource, then the player will have to discard their zero cost events as well!

Threat level: Mid to High. The average case for Fragile Thoughts is probably a bit lower than the one for Corrosion, but the ceiling is just as high. This is a card that can easily shred someone’s hand.

Dealing with it: Most investigators do not need to be terribly afraid of this card, but some (Like Nathaniel, Sefina or Preston) are built around using events and can easily get their hand and in turn their whole gameplan destroyed by this card. Keeping a high cost event (Dynamite Blast, Hot Streak) in hand to feed into Fragile Thoughts would be preferrable, but of course not everyone actually runs a card like that.

What it does: After failing a willpower test, the investigator loses an action and a point of sanity. The difficulty of the test scales with the shroud value of the investigator’s location.

My take: Sanity is in short supply in the Unspeakable Oath, so the card has a bit more punch there than it has in Last King. That being said, this is a fairly tame card, losing one of your actions to dealing with whatever the Mythos phase brings is basically expected. Even a Swarm of Rats will cost an action.

Threat level: Very Low to Low. A very basic effect and below average in impact.

Dealing with it: There’s not really anything in particular that can or should be done about this card. Usually drawing this should be preferable to drawing anything more volatile.