Agents of Yuggoth

My take on this set: This set does add yet another layer to the concealment mechanic of the campaign. It is a set that is particularly punishing if players keep a lot of enemies in the shadows. But on the other hand it rewards players who proactively seek out anything in concealment. The reason for that is that all cards in this set are only dangerous if they are allowed to stack up with other concealed cards, so if you have a handle on what’s in the shadows, this set of cards isn’t going to do much to you.
It is a bit weird then that this set is used during On Thin Ice, since that scenario doesn’t even have that much concealment in it. Sure there is some, but On Thin Ice also features Outsiders and hollowing at least as much. In turn, Eclipse will be a rather tame or even free draw many times during On Thin Ice.
This is much different during Dealings in the Dark of course. That scenario is all about concealment, with many cultists and coterie members hiding out in the shadows at the same time even at low player counts. Eclipse can be a very threatening card in that scenario.
I think this is a well done set that succeeds at giving concealment more urgency. My one critique would be that mechanically it would’ve fit better into Dancing Mad than into On Thin Ice.

What it does: Emissary of Yuggoth is a moderately sized enemy with completely average stats and damage/horror values. It does however have Concealed 2 and is able to attack enemies while being in the shadows, gaining a free attack at any investigator that discovers a clue at a location with a concealed mini-card at it.
Once exposed, it gains Hunter and Massive as relevant keywords.
Defeating the Emissary awards a victory point.

My take: This enemy breaks the mold in a couple of ways, making it a bit of a weird card. It’s the latest in a series of Victory enemies from Agents of X sets but of all them it’s the one that poses the least problems in a straight up fight. Instead it is mostly notable for harassing the players through the threat of attacking from the shadows.
Hunter is not really all that relevant on a card with Concealment, as it will only come into play if exposed, then evaded. And considering that Emissary isn’t terribly hard to defeat and also rewards doing so with a VP, running away from it isn’t a great plan in the first place.
I don’t find this enemy particularly scary. It’s relevant sure, but we’ve come to expect much worse from the Agents of X sets.

Threat level: Medium. Concealed 2 makes it stack up well with other concealed cards. You will usually be able to avoid triggering its Forced ability, but of course playing around it is already a hindrance.

Dealing with it: Flush it out of the shadows and kill it. Since its not terribly threatening once engaged, this should be your first plan when facing the Emissary. This enemy is worst if it’s allowed to stack up with other concealed cards, so in preparation you should keep the number of enemies in the shadows under control. Especially during Dealings in the Dark that is of course easier said than done, but it does align with what you probably want to do in that scenario anyways.

What it does: Tenebrous Eclipse stays around for one round. While in effect, each investigator can only expose one mini-card per round, severely limiting their ability to go after the enemies in the shadows. If multiple of this card are in play at the same time, only one is discarded at the end of the round, making this limitation stretch across several rounds.

My take: Much more than the Emissary, this treachery can be very annoying. It his highly contextual. If there are no enemies in the shadows when drawing this, it’s even a freebie. But if you are already behind on exposing the enemies and have multiples in the shadows, then Eclipse will make the situation that much worse by limiting your ability to find them.
Just like Deep Dark from the Forgotten Age, Tenebrous Eclipse also gets a lot worse if you draw multiples in such a way that its effect covers multiple rounds in succession. Especially in full groups this happens way more often than one would like.

Threat level: Medium. Highly dependent on the board state. This card doesn’t cost you any of your resources, actions or cards so as long as you can play around it, you don’t actually lose anything. However, the way it can limit your options can absolutely be very punishing.

Dealing with it: Since it’s a card that depends on the current board state, dealing with it proactively instead of trying to clean up after it is the best way of meeting this treachery. With three of them in the deck, you can expect to draw them multiple times per scenario, so working towards minimizing their effect is going to pay off. Mostly that means going after enemies in the shadows straight away and not letting them stick around for too long. You will want to avoid situations in which you have three or more enemies in concealment and then not be able to do anything about them because you drew two Tenebrous Eclipse in back to back turns. During On Thin Ice that wouldn’t even be that bad, since it’s not actually all that focused on concealment and just has a splash of it. But in Dealings in the Dark that situation could easily make you struggle to keep up with the cultists.