This has been updated to match my "Signature Weakness Project." I have done my best to make sure that the original content isn't altered too much, out of respect for any comments.

A very rare "attach to the agenda" signature weakness. Looking at the two elements, the effect and the discard condition, we get:

The effect: Attach to the current agenda. Carolyn takes a direct horror. If all horror generated isn't healed by the end of each round, add a doom to the agenda. While this weakness, like many, is pretty situational, it's still nasty in most settings, and can be scenario-ending. You never want to lose turns to "add doom" effects, and, unless you draw this at the Witching Hour or draw it on a low-doom agenda (or are lucky enough for all investigators to avoid or soak any horror), it's likely to cost you a couple of turns, especially since, if drawn during upkeep, the horror Carolyn takes from the card will generate a doom before anyone can heal it. The team can stick close and have Dr. Fern desperately heal instead of advancing the act, but that is its own kind of tempo hit, as is having everyone try to maintain their horror soak and own healing abilities. The only real mitigation is getting better horror healing (which Carolyn does as she gains XP) and have everyone mind their soak. This card is made slightly worse by the annoyance of having to track who gained horror each round.

The discard condition: Advance the agenda to which it is attached.

All in all, this is an above average signature weakness, maybe even way above in scenarios with high doom agendas.

Box vs book Since Foolishness, while cute as a button, is not the best Ally in the game, and this is a pretty harsh weakness, I suspect most players will choose the "box" over the "book" or including both in building a deck (the two signature weaknesses have a really bad synergy). Which is a pity, because I like the Dreamlands flavor.

Pretty much what you want in a signature asset -- it makes Dr. Fern more of what she already is, adding a little card draw and the option for quicker healing. The test puts the breaks on a bit, but Carolyn should be able to pass it most of the time in Standard. At higher bag difficulties, you probably want to avoid unnecessary tests due to those nasty special tokens, so this would get used for the reaction ability most of the time, I expect.

This is paired with the less troublesome of Carolyn's signature weaknesses, and is definitely easier to put to use than Foolishness, so most plays will stick with the "box" over the "book" sets.

This has been updated to match my "Signature Weakness Project." I have done my best to make sure that the original content isn't altered too much, out of respect for any comments.

A very investigator-focused signature weakness. Looking at the two elements, the effect and the discard condition, we get:

The effect: At the start of a campaign, this card eats up about 4 actions, blocks Dr. Fern from useful horror healing, and messes up her economy for at least 2 turns. Which is pretty severe. The only way to mitigate it is get better horror healing, which comes with XP. A the start, horror healing acceleration is limited to Logical Reasoning, Soothing Melody, and Hypnotic Therapy. As XP accrues, 2 point healing gets reasonably common (which can be boosted to 3 with Hypnotic Therapy), and Logical Reasoning (4) can clear it in one action, if you have 2 clues. Ancient Stone:Minds in Harmony (4) can even do it as a reaction, assuming you can get 4 secrets on it and draw 4 cards. I personally like the idea of Kerosene, just because it seems like such an extreme therapy.... In most cases, a reasonably experienced Carolyn will clear it in 2 actions, which is not too bad.

The discard condition: Heal 4 horror off it, which, as noted above, gets easier as XP accrues.

When you think about it, this is an average signature weakness, maybe barely above average at low XP.

Box vs book It's better than To Fight the Black Wind in most situations, and it's paired with a much more useful signature asset, so most player will chose the "box" pair rather than the "book" set or doubling up, especially since having this and the Black Wind out at the same time are a horrible synergy.

This has been updated to match my "Signature Weakness Project." I have done my best to make sure that the original content isn't altered too much, out of respect for any comments.

Another weakness aimed square at the investigator's special ability. Looking at the two elements, the effect and the discard condition, we get:

The effect: You can't add assets to the play area. Dexter likes putting assets into play, it's kind of his jam, and this card stops that hard (see the note at the end about interpretation of the text). This is another very situational weakness. While you are never happy to see your demonic "friend" show up, sometimes it absolutely wrecks you, while other times you can limp to the end of the scenario without too many regrets. In some ways, it the exact opposite of Searching for Izzie or Cover Up, in that, if you have to draw it, the last turn or two is when you want to do do so. As usual with the to remove weaknesses, this has much greater impact in solo than multiplayer.

The discard condition: Spend two actions.

Taken together, this is a below average signature weakness, maybe average in solo.

Box vs book While this weakness is probably less annoying overall than Occult Scraps, the "box" signature weakness, neither is too terrible, making Dexter one of the investigators who should think seriously about including both sets of signature cards in one deck.

Note: The card text is a bit puzzling, since it seems obvious that the meaning is that Dexter cannot add assets to his play area. However, the Rules Reference makes clear that "play" and "put into play" are completely separate actions, suggesting that Dexter is only forbidden Anna Kaslow and Sleight of Hand, which would require creating an entirely new tier of "This is barely a weakness." When asked about it, MJ Newman responded "The intent is that you cannot do either—play assets or put them into play—the idea being that playing something does in fact also put it into play. However, as you rightly point out, that’s actually incorrect as far as the rules are concerned, so we’ll be updating the rules to reflect this in a later edition of the FAQ." So, Yaztaroth prevents any assets for Dexter.

Comparing this with the "book" signature asset, Molly Maxwell, it's hard to say which one is better, since they do such different things. This is a nice example of a card tailored toward its investigator. It's cheap and adds a valuable boost to what Dexter wants to do -- put Assets into play, use them and then discard them. He can drop a Flashlight at the very start of his turn, assuming he has another Asset to discard, investigate 3 times with an effective 4 and -2 shroud, then start the next turn playing a .41 Derringer to blast away with 7 against an enemy who popped up during the Mythos Phase. It's not flashy, but it's better than it looks at first glance (which is pretty thematic for a magician).

Since it really has no competition with the "book" signature, Dexter is pretty well placed to take both.