Left Behind

My take on this set: This is the closest thing that Edge of the Earth has to a cultist set, but they do work quite differently to what we are used to. The most important part is that defeating these won’t remove the doom from the board, it merely moves those tokens to the locations where they can’t be interacted with any further.
That can make them quite the pain to deal with if nobody with the correct skills is around to make these lost explorers snap out of it.
This set is used only twice during the campaign, during Ice and Death #2 and Fatal Mirage. Since both of those are optional, it is possible to finish Edge of the Earth without ever using this set. On the other hand, you might visit Fatal Mirage multiple times and be exposed to Left Behind more often. At least Fatal Mirage doesn’t have Ancient Evils which stacks up with the doom from this set dramatically in Ice and Death #2.

What it does: Lost Researcher spawns at an empty location, with a doom token on it. This possessed scientist is easy to defeat, however doing so will merely move the doom to the location. To get rid of the doom, the players need to instead Parley with an action, passing a willpower or intellect test. Should they succeed, a doom token is flipped into a clue token under the player’s control.
If no doom tokens are remaining following a successful Parley, the Lost Researcher is discarded.

My take: Of the two Possessed enemies from the set, this is the one that is less troublesome just because of the fact that there’s two different skills available here to test against. And one of them, Intellect, is pretty much a base requirement for someone to have anyways, so you should generally be able to handle this guy fine. Now, he does spawn at an empty location, so some movement is going to be involved which can cost some actions but you are free to spawn him as close as you can to minimize the effort. And hey, at least you are getting a clue for your trouble. That can save an action somewhere else down the line. All things considered i think this guy is fine… for a cultist. Anything that has doom on it is still making at least some impact of course.

Threat level: Low to Mid. Is not going to be too difficult to deal with, but you can’t ignore it either.

Dealing with it: Killing it doesn’t help in any way, so you will have to go through the Parley option if you don’t want to see the agenda advance prematurely. Willpower and Intellect are usually easy enough to get by, even if it is just in short bursts thanks to some Guts, Perception or whatever other icons you can scrounge up.

What it does: Frenzied Explorer spawns at the player’s location, with a doom token on them. Like with the Researcher, defeating the Explorer is no huge issue, however that would only move the doom to the location. To get rid of the doom, the players need to evade the Explorer by 2 or more which on success will have the same result as for the Researcher: The doom is flipped to a clue and if it was the last one, the Explorer is discarded.

My take: While Intellect and Willpower are usually readily available in a group of investigators, agility is sometimes much more of an ask. Especially since you need to oversucceed on your evasion here, putting the effective difficulty to flip the doom into a clue at 4. That’s not trivial at all.
There are certainly some high-agility investigators who will be delighted to pull this card instead of something else, as they can just spend an action and do their test to not only deal with their encounter card for the turn but also pick up a clue. But the majority will have to either hope that someone else on the team can bail them out or they might have to settle for just barely evading the Explorer. Or possibly killing him. Either way will not remove the doom from the board, though.
Following Innsmouth Conspiracy which already put a heavier emphasis on agility, Edge of the Earth has been increasing the pressure on that skill even more and the Explorer is an example of a card that does so without explicitly calling for an agility test.

Threat level: Mid to High. This one is actually hard to classify. If you don’t have someone that is able to do the +2 evade, then this is basically an Ancient Evils that also costs an action to evade/kill. Otherwise, it’s an almost free clue. Very low floor, very high ceiling.

Dealing with it: The way to deal with it is printed on the card and there are very few ways to get around it. It doesn’t explicitly call out agility, so the doom can be flipped into a clue by a Mystic with Mists of R’lyeh, Sword Cane, etc.
But anything that automatically evades, like Decoy or Daniela’s investigator ability won’t do you any good here.
If none of your investigators are up to the task of turning this guys doom into a clue, then you can enlist Eliyah’s help (as long as he’s still alive and not missing…). He will offer 3 tries at agility 5 which can get the job done. I’d still plan on throwing a couple icons into those tests, though.
As a final note: Going for the over-success and then pulling from the bag in a way that makes you succeed only by 1 or 0 is just the worst thing ever, as it means you have to spend another 2 actions on engaging and evading the already exhausted enemy again. Feels absolutely awful.

What it does: Abandoned to Madness attaches to the nearest Possessed enemy, adding a doom token to that enemy in the process. While attached, that enemy is more difficult to attack, evade or parley.
If no Possessed enemy is in play, the player has to search the encounter and discard pile for one, spawn it and then attach Abandoned to Madness to that card. Note that this will NOT put a second doom token on the enemy you just searched up.

My take: I am not in the habit of keeping enemies with doom on the board, so whenever i draw this, i am likely going to search up an enemy from the deck. For this one, i will always search from the deck instead of the discard pile, to reduce the amount of enemies in the deck. Which one to pull is going to depend on what you can most easily handle. Chances are, it’s going to be a Researcher because there’s just too much that can go wrong with the Explorer especially when its doom->clue difficulty is raised to an effective 5 by this card.
If you do have a Possessed enemy already on the board, this is much worse on an Frenzied Explorer than on a Lost Researcher. Removing two doom from an Explorer in one turn requires spending an extra engage action between attempts, while you can just double parley the Researcher without such extra hurdles.
In Ice and Death #2 there’s a chance that Abandoned to Madness attaches to one of the missing crew members, since those also have the Possessed trait. The effect ranges from annoying to barely noticeable. If one of them is already in play during Mythos, you will likely will have already worked towards parleying them last turn and now only need to get the final couple counters before they enter the victory display. Increasing the difficulty on the parley can hinder you a bit with that, especially Roald and Claypool who are already very difficult to parley. Others, not so much as it will only increase the difficulty from 2 to 3. Some are even completely unaffected. Of course they will get a doom. But that goes away as soon as the card goes into the victory display and you will typically not have the explorers around for 3+ turns.

Threat level: Mid. Acts as extra copies of the other two cards from this set. Can get nasty if it results in two doom on the same enemy, but you do get to choose what exactly this card searches up most of the time.

Dealing with it: When you draw this card, it’s already too late to deal with it. This card is a reason to be very proactive about the two Possessed enemies. Don’t let them linger, try to remove them before drawing Abandoned to Madness. This card is much weaker when it doesn’t put an additional doom into play.