TL:DR, avoid like the plague unless your options are limited or your name is Agnes Baker.

The faction is one of the two most resource starved groups in the game (the other being ), powerful economy cards are great but their cost may just be too great for you to pay for. Forbidden Knowledge is one such card, for the vast number of characters the horror is just too great and they perish quickly, Agnes Baker however LOVES this card.

• 4 Resources is good, 4 horror is BAD, from what we can surmise at the moment two points of horror heal is worth a card and two resources, I.E the trade here is very inefficient, not to mention the greatly increased risks in the scenario. Almost no characters have the horror tank to endure this and those who do might want to look at other options, like Uncage the Soul and Emergency Cache, both of those grant similar resource benefits and at no horror cost.

• Secrets rarely combo, there are just 2 cards in the game that interact with this resource type and those cards are bad on their own.

So, it kills you and compares terribly to other cards, so why does Agnes like it? Heck, this card might as well be her strength card, and a 2-off at that! The main reason Agnes loves it is the fact that she can trigger the trigger outside her normal turn. With generous application of spells and being buried in enemies something like this might happen:

Trigger Forbidden Knowledge in the Mythos phase, shoot something with Shrivelling and pull a , get punched in the enemy phase taking another horror. That's 3 triggers on her ability in one turn which is another 3 points of damage on top of her Shrivelling, enough to kill bosses in solo in one round! Of course, typically, you'll perhaps not draw a special token with the Shrivelling or draw horror inflicting effects in the mythos phase, Forbidden Knowledge is very flexible so you can usually adjust accordingly. To be able to pull off combos like these you absolutely need Fearless and might need to forego a copy, or even both copies, of Arcane Research.

Pros:

Amazingly versatile: if all copies of Soothing Melody are drawn, you can heal up to 3 damage and 3 horror or 6 damage/horror from any investigator and/or ally at your location in any combination. This makes Soothing Melody the equivalent of 3 copies of Emergency Aid, or 3 uses of First Aid(3) with added card draw.

Amazingly versatile part 2: with cards such as Moment of Respite and First Aid(3) you can “waste” them by not being able to use the effects fully, for example: only healing 2 horror for Moment of Respite, or only healing 1 horror from your ally and not needing the health heal from First Aid(3). By contrast, Soothing Melody has such a variety of potential targets that it’s effect will never be wasted.

Cons:

Has the common healing card disadvantage of requiring 2 actions to play and then use, although this is mitigated by the added card draw.

Both Hallowed Mirror and Soothing Melody take an action to play, meaning you need to be disengaged from enemies if you don’t want to eat an attack of opportunity to the face.

Hallowed Mirror is extremely weak to “asset hate” cards, such as Corrosion in Path to Carcosa, requiring secondary asset protection (and backup horror mitigation if removed before asset protection in place) to ensure all Soothing Melody cards in the deck can be drawn.

Guardians usually use the accessory slot for sanity protection, Hallowed Mirror denies the slot until removed from play, requiring Relic Hunter (3xp) to gain a second accessory slot.

Hallowed Mirror is only a singleton card: sources of card draw or deck searching are needed to play it reliably, as well as the resulting Soothing Melody cards.

Similar to Emergency Aid, Soothing Melody is drip-feed healing compared to First Aid’s consistency of being able to tap when required once played.

Soothing Melody is outclassed by Moment of Respite(3) on a 1:1 comparison, as well as the combo of First Aid(3) and Emergency Cache(3), although by this point you’ve spent 6xp on two cards to outclass a single level 0 card.

Pretty sure you can stick this in a Backpack & forget about it!

Sadly, Backpack specifies non-weakness items.

But conveniently, there's still Ever Vigilant and Dr. Elli Horowitz!

Haven't gotten far in Carcosa yet, but seems pretty mild overall. Basically just -1 action if you have nothing else to help.

(Thank you @Zinjanthropus and @Death by Chocolate for comments)

Lure is part of a suite of Survivor cards that attach to locations and affect enemies at that location.

Hiding Spot, a level 0 card, makes non-Elite enemies at that location aloof. Hiding Spot can be used reactively to prevent Hunter enemies attacking you or proactively to make sure that any non-Elite non-Massive enemy that spawns at your location doesn't engage you. It's Fast, which makes it a highly efficient card.

Snare Trap, a level 2 card, can be used to exhaust a non-Elite enemy that enters your location and prevent it from straightening in the next round, which includes spawning at that location.

Lure (1) and Lure (2) control enemy movement during the enemy phase. Unlike Snare Trap and Hiding Spot, there is nothing preventing them from affecting Elite enemies. This is very powerful in scenarios with Massive Elite Hunters, particularly The Devourer Below. Lure (1), however, always had the problem that you could only use it at your own location, which made it difficult to use. Lure (2) , however, can be dropped at a connecting location, which makes it a lot more playable. If nothing else, you can always put it in a Hunter's location so that the Hunter will stay where it is.

So, what the upsides and downsides of the card? Well, one downside is that the card only lasts for one round. Hiding Spot and Snare Trap can be dropped in a space you intend to spend a long time at, and will protect you until needed - that is until an enemy spawns or moves into your location. They also protect you from enemy movement during the Mythos phase, which neither version of Lure has any effect on. It won't, in other words, affect an enemy like The Experiment.

An upside, one quite relevant to The Circle Undone, is that Lure (2) will protect you from Aloof Hunters such as Whippoorwills and the Nether Mist.

A final upside is that by playing it you can cluster multiple Hunter enemies in one location, allowing you to set up a future area attack or area evade.

So what investigators are best suited to play it? Well, Wendy Adams can play it every turn if she gets her Amulet out provided she avoids discarding or playing any Event cards in the Mythos Phase, The timing of the end of round discard from play makes it unlikely to be removed from the game by Abandoned and Alone. Hiding Spot can also be played indefinitely, but playing it in successive turns is often not much help. William Yorick can use it to set up Dynamite Blast, and Agnes Baker can similarly combo it with Storm of Spirits. Any character who can play this card can play Survival Instinct, Cunning Distraction and Impromptu Barrier. There is currently no Investigator who can take it and the Mk 1 Grenades or the Rogue version of the .45 Thompson.

So what does the future hold for this card? Well, cards that appear late in one cycle are often hints for what's coming in the next cycle, and the design of Luke Robinson, as well as this card and Bait and Switch (3), all suggest that dealing with enemies at connecting locations might be a serious consideration in The Dream Eaters.

1. Double, Double.
2. The title of this card is a single word, doubled.
3. The cost of this card is 2 x 2 = 4.
4. The level of this card is also 4.
5. Because it is exceptional, you pay double the XP to buy this card.
6. Ironically, because it is exceptional, you can only have 1 in your deck.
7. It has 2 commit icons on it.
8. Not counting the duplicating coin in the middle, there are 16 (or 2^2^2) intact coins on the artwork.
9. It lets you play any event twice.
10. Aside from the keyword, the flavor text and rules text each take up 2 lines (4 total)
11. It takes up an Arcane slot, of which you have 2.
12. It is card 320 (2^5 x 10, note 2 x 5 = 10) in "The Circle Undone" cycle.
13. It is in "Before the Black Throne", the Mythos pack containing the 8th Scenario this cycle.
14. "The Circle Undone" is the 4th cycle of Arkham Horror LCG.
15. The set symbol for "The Circle Undone" shows a severed figure 8, which contains 2 loops.
16. The Rogue class symbol has 2 loops intertwined as well.

Alright enough of that. Clearly this is a Sefina Rousseau card. Get some extra value from The Painted World, a card that normally is restricted to three plays a game. Because Painted World becomes a copy of whatever card you play it as initially, I'm fairly sure you'd have to play that same event again if you Double, Double it, instead of getting to change targets, but that's okay. There's a lot of cards worth doubling: Drawn to the Flame, Sneak Attack, Backstab, Swift Reflexes, Narrow Escape, Hot Streak. Some events wont benefit well from this (Ward of Protection, Drawn to the Flame on lower player counts, Sure Gamble), but overall you'll probably get more than enough value out of it to make it worthwhile. Hot Streak is already a very powerful Sefina card when combined with Lola Santiago, so doubling it seems strong. Speaking of Lola and Hot Streak, Preston Fairmont likes this card as well. Preston relies on events to do anything (Intel Report, Decoy, Small Favor, "Look what I found!") so this could be very valuable action economy for him. Oh, and there's also Ace in the Hole.

That said I'm not fully sure how some of the timing would work there. Could you use it to discover four clues with "Look what I found!"? Could Sefina double Think on Your Feet and move two locations instead of one, or can she not double the card since she didn't draw another enemy? I'm not sure how the timing works here. Still, I'm sure there will be plenty of targets for this card. Two paragraphs.