- If you draw Obscuring Fog while there's another copy already attached to your location, discard the copy that you just drew.
Revelation - Attach to your location. Limit 1 per location.
Attached location gets +2 shroud.
Forced - After attached location is successfully investigated: Discard Obscuring Fog.
Just wanted to take a moment to appreciate how rarely this card screws you over. So often it spawns on a location that's already out of clues, or had none to begin with, or even already had a copy of fog on it, so does absolutely nothing. The only problem with that is that it stays out of the deck so it's not around to provide you with any more dead draws in the mythos phase, Its worst case scenarios are blocking a high shroud victory point location, and even then it's about worth a perception or flashlight charge. Adds +2 difficulty to something the seeker is already doing, and doesn't even block you from picking a clue.
I suppose it's a lot worse in higher difficulty levels, and it does block the strategy of flashlighting a location down to 0 shroud. But even in those scenarios, you have all the methods for picking up testless clues that you can just save for when this gets attached to a location.
Overall, I just can't think of any treachery that's shown up in a nail-biting mythos phase where certain cards will just force a loss, that gives as great a sigh of relief when it shows up instead of those. Though perhaps that's just because it's in so many levels.