Forgotten Ruins

My take on this set: The three cards from this set are all very different in their mechanics, but they all work towards making the doom clock more pressing. They either add doom to the locations or, in the case of Deep Dark, stop players from advancing their goals and lose turns that way. This is a potent set of cards that has a lot of influence on the scenario they are in. A special case is Depths of Yoth, which makes Ill Omen and Ancestral Fear both considerably less scary, but in turn players are impacted by Deep Dark a lot more there.
This is a good set. I appreciate how different these three cards are while still forming something coherent in the end. All three are also terrifying in their own ways.

What it does: For the rest of the turn, each investigator can only discover one clue from each location. The treachery is added to play next to the agenda for that turn. Should more than one Deep Dark be revealed at the same time, only one of them is discarded at the end of the round.

My take: This card is consistently a big source of grief for me despite only playing with two investigators. In a full group, this must be even worse. In Doom of Eztli, this card is only an annoyance, as it won’t stop you from exploring further. Depths of Yoth and Heart of the Elders do require you to remove all clues from a location before exploring, though. Meaning that Deep Dark stops the group right in their tracks, forcing them to find something else to spend their actions on productively. This is easier said than done, especially if multiple Deep Dark turns follow each other, something that becomes more likely with more players. Meanwhile, this also gives any Hunter enemies a chance to catch up with the investigators.
Note that this prohibits finding more than 1 clue by any means, not only from investigations. So unlike with many other cards of its ilk (like Locked Doors) this can not be bypassed with Working a Hunch, Drawn to the Flame, etc.

Threat level: Mid. While it doesn’t cost the investigators anything but options, and only for a turn, it is still a very relevant delay that matters a lot due to the context of the scenarios it is in. Only during Doom of Eztli this matters a little less.

Dealing with it: This punishes groups that rely on a single Seeker for clue discovery the most. Otherwise everyone can at least grab one clue to keep working towards clearing a location.

What it does: The player who drew it (and only they, thanks to Peril) chooses a location that has at least one investigator. That location has a doom added to it and all investigators there take one horror.

My take: Adding a doom is of course not something we want to do ever. This even adds it to the location, which means that doom token persists through the advancing agenda in Doom of Eztli and Heart of the Elders. That horror dealt is just the final injury added to the insult. Terrible card. Depths of Yoth removes the doom when reshuffling the locations from one depth level to the next, so at least that removes a lot of the sting from this card. The agenda doom thresholds are much lower there as well, though.

Threat level: Very High. This is Ancient Evils on steroids.

Dealing with it: A good amount of locations have some special effects when there’s doom on them, so that should go into the consideration of where to put the doom token. Aside from that, there’s not much more to do here. It’s Peril, making it hard to cancel, so you will very likely end up losing at least one turn for everyone here.

What it does: Ancestral Fear, another Peril card, asks the player who drew it to make a choice: Either put a doom on the location they are at or add Ancestral Fear to the victory display. While Ancestral Fear is in the victory display, it counts for one Vengeance.
Independent of what the player chose, Ancestral Fear does also surge.

My take: Right. If Ill Omens is Ancient Evils on steroids, then this is Ancient Evils on steroids with Surge. Unless… unless you take a vengeance. Which i will almost always do, unless i am already in my last few turns. The thing with putting doom on the location is that this is not also quite the bad effect but it also keeps Ancestral Fear in the encounter deck. Meaning you can draw it again later. And if that happens you will wish that you added it to the victory display the first time already.
In my opinion it is fine to get 10 points of Yigs Fury before reaching Depths of Yoth. Drawing Ancestral Fear a couple of times easily fits that budget and leaves room for the occasional exploration at a Resentful Wilds or throwing down the occasional stick of dynamite down a Snake Pit…
The only redeeming quality of this card is how it’s near blank during Depths of Yoth because you are past the main payoff for Vengeance. But of course it still surges, so that’s not even something that would offer a moment of peace.

Threat level: High. Surge with a relevant effect. You are never happy to see this.

Dealing with it: Just take the Vengeance. Nobody needs surging Ancient Evils in their life, unless you are about to finish up the scenario anyways.