Outsiders

My take on this set: This is a potent set. Prominently featured in Riddles and Rain, it introduces the mechanic of hollows: set aside cards that are referenced by other effects later. Outsiders is one of the bigger sets in Scarlet Keys (most sets are 4 cards), giving this a decent presence in the encounter decks it is part of.
All cards in this set have a noticeable impact, there are certainly no freebies here. Even the Paracausal Entity which appears like a rather standard enemy at first with an ability that doesn’t have any immediate impact does serve as a setup for other hollow effects down the line.

What it does: Paracausal Entity is a small-ish enemy that does still exert some board presence when it comes up. While it can be defeated in one swing from most weapons, it’s at least not trivial to defeat or evade. It’s a Hunter that hits for a damage and horror each. Whenever it engages an investigator, that investigator has to remove their top card of the deck from the game, as a hollow.

My take: There’s three of them in the set, so these are a fairly common sight whenever this encounter set is used. Unlike most things in Scarlet Keys, these go down from just 2 damage, so that immediately makes them a bit of a breather when compared to what else the typical encounter deck has to offer. Removing the top card of the deck doesn’t do anything relevant by itself (unless you rely on things like assembling Pendant pieces, which is just a bad idea in this campaign in general), but it does serve as setup for Substance Dissimilation and a couple other effects related to hollows.

Threat level: Low to Mid. They aren’t trivial, but as far as enemies go, these are on the weaker side.

Dealing with it: Killing these shouldn’t pose much of a problem. More interesting is taking note of what card they removed and consider what that means for Substance Dissimilation or any other hollow effects in the scenario.

What it does: At 5 health, the Apocalyptic Presage is a chunky enemy that requires some effort taking out. It also sports enough of a fight value that reliably attacking it requires a dedicated attacker. Its evasion is low which can be used as a fallback if the Presage is too difficult to deal with right now. As soon as this enemy enters play, all investigators need to set aside on of their cards in hand or play, as a hollow. Defeating the Presage rewards a victory point, but the investigator who defeated it can instead chose to discard the Presage and return up to 3 hollows to their owner’s hands.

My take: This thing can be a huge roadblock. And it can come down from the encounter deck at any point (at least Riddle and Rain has the Outsiders set aside for the first couple turns, though). It reminds me a lot of Deep One Bull that way… and the Bull actually has the same statline as Presage, so they do indeed cover pretty much the same bases.
Unlike the Paracausal Entity the cards that are getting hollowed come from the players hands, so they do limit the players options at least somewhat. The effect also hits everyone and not just the one who engages the thing.
Defeating it gives the option of recovering 3 hollowed cards, but it’s going to be rare that this option is worth it. Getting the equivalent of a “Draw 3” is certainly nice, but giving up the victory point is a big ask.

Threat level: High. This enemy is always going to be relevant when it shows up.

Dealing with it: Compared to the Paracausal Entity, this one at least doesn’t keep hollowing cards whenever it gets engaged, so evading it for a turn or three is a decent option to keep the enemy away from you while you finish the scenario. Of course you’ll optimally want to defeat the thing to claim your victory point, but especially early on this might just not be in the cards. Something to note here: Presage is not an Elite enemy, opening it up to a bunch of player cards that can deal with it… including Waylay which is a great way to capitalize on the low evasion value of Presage.

What it does: Each card in hand or in play that matches a card set aside as a hollow is discarded. Additionally, for each set aside card matched this way, a damage and horror is dealt to the investigator.
Should no cards be discarded by Substance Dissimilation, then the player has to hollow one of their cards from their hand or play.

My take: This card is potentially ridiculous and can devastate a board and/or hand while also dealing a lot of damage and horror at the same time. Its effect is very swingy with a very high ceiling, but of course it will on average not hit quite as hard. But still, having to discard a card from play and being dealt a damage and horror is already ahead of the curve in terms of what treacheries usually do.
I am not a huge fan of this card existing. If there were more ways to interact with the hollowed cards, then this would act as an interesting threat to play around. But the primary way of getting cards hollowed is usually drawn blind from the top of the deck without player choice and recovering cards from there is also incredibly rare. So this plays out as a very random card that just punishes you hard for things you had no influence on without much in the way of combating it.

Threat level: High. Unpredictable and with a high ceiling, this is a dangerous card.

Dealing with it: If the Outsider set is in the deck, this is a card to keep in the back of your mind. For one, anytime you get a choice of what to hollow (like on Presage) you will usually not want to pick cards you have a copy of on the board because then you are opening yourself up to having to trash those cards later. Hollowing cards that already have a hollowed copy is safe in this regard. The other thing is that if you have a lot of hollowed cards right now, drawing Dissimilation can potentially deal a large chunk of horror and damage to you, so if you are already near your stamina or sanity threshold, it can be worth to aggressively get rid of any copies of hollowed cards in your hand or from play by committing them or overwriting them with other cards in their slot so you don’t get nuked unexpectedly.