The Stranger

My take on this set: Three of the scenarios use this encounter set to increase the chances of having the Man in the Pallid Mask enter play. To that end, The Pale Mask Beckons acts as a second copy of the weakness enemy that is shuffled into the encounter deck.
The other card, Marked by the Sign, is a potent horror dealing treachery. It swings from almost okay to bad depending on the presence of the Man in the Pallid Mask by threatening to deal direct horror to a player. Thus, it acts as a way to punish the players for not seeking out the stranger to remove him from the board.
There are two places where the campaign cares about the number of checkmarks the players collected for defeating the stranger: During setup of Phantom of Truth, having 4 or more checkmarks awards players with an additional tick for either Doubt or Conviction. And during Dim Carcosa, the first agenda starts with extra doom depending on how well the players followed this sidequest. This encounter set doesn’t help a whole lot with the first time, as it is only used once before Phantom of Truth. The extra three possible occurences before Dim Carcosa are appreciated, though.
The cards from this encounter set all deal horror, which usually finds ways to stack up with that from other encounter cards (from Hastur’s Gift or Agents of Hastur, for example), but not to the extent that Unspeakable Oath and Dim Carcosa do.

What it does: Marked by the Sign asks the player to pass a willpower test. If they fail, they are dealt 2 horror. The card has Peril, so no other players are allowed to help with the test. Should the Man in the Pallid Mask be in play, this card is upgraded in two ways: For one, the test is more difficult. And secondly, the horror becomes direct horror instead.

My take: One of the very few encounter cards that deal direct horror, bypassing all the usual soaking assets that players run to extend their sanity. The Carcosa campaign already puts an above average amount of pressure on sanity, so those two points of horror are usually very relevant and making it direct is only going to increase the chance for this card to threaten the player. The difficulty of the test is either 2 or 4, which is quite the dramatic swing in terms of who can expect to succeed at passing it.
The saving grace for this card (and this set, really) is the selection of scenarios in which it is used. Last King, Phantom and Black Stars are all scenarios that do not necessarily pile on the horror as much as for example Unspeakable Oath, Pallid Mask and Dim Carcosa do.

Threat level: Medium. Direct horror can be a real pain and just knowing that this card is around takes some of the leeway that players have in dealing with horror away.

Dealing with it: More than usual, consider preserving your investigator’s own sanity, even if it may lead to discarding an ally or asset. Of course, this card is also a good motivator to go and deal with the Man in the Pallid Mask once he shows up.

What it does: The Man in the Pallid Mask is put into play from his bearer’s deck. If he already was in play, he does an immediate attack against everyone instead.

My take: Being attacked by the Man in the Pallid Mask equals to being dealt one point of horror, which isn’t nothing exceptional by itself. But it does of course hit everyone in the group and it does stack up with the other card in this encounter set.
The card’s most important function is summoning the Man himself, though. In that function, the treachery isn’t even bad, it basically works as drawing a card for the player (even if it is a weakness, that is one weakness less to draw another time) and works towards getting those checkmarks for pursuing the Man in the Pallid Mask.

Threat level: Low. Most of the time, this card is beneficial. If it isn’t, it’s still not too bad.

Dealing with it: Nothing much to do here. This card is more or less part of the scenario mechanics, giving players another chance to draw the unique weakness enemy that they want to meet and defeat.