
Pointing out potentially obscure interaction (if it's already been discussed, my b):
The ability on "Fool me once..." is entirely optional. The means you can lock away nasty treacheries like Ancient Evils forever to decrease their appearance from the encounter deck at all. Then in the final stages of the scenario you can always choose to cancel it anyway if you'll finish before reshuffling the encounter deck.
The reason I'm bringing this up at all is because my group has found that Syzygy is just an absolute nightmare to handle. Since it is a Peril, your Ward of Protection or A Test of Will packing friends won't be allowed to cancel it. If you have a way to ensure that you're the one to draw Syzygy (say... First Watch or Scrying) you can ensure one or both of them are completely removed from the scenario for its duration.
Notably the reverse also applies. "Fool me once..." cannot cancel Perils drawn by your allies despite its card text.
"Peril is a keyword ability.
While resolving the drawing of an encounter card with the peril keyword, an investigator cannot confer with the other players. Those players cannot play cards, trigger abilities, or commit cards to that investigator's skill test(s) while the peril encounter is resolving."
Anyway, there's plenty of encounter sets that include only one or two copies of completely disgusting treacheries that can be completely negated for the remainder with some luck and/or some encounter deck manipulation. Give it a shot!