Used in: The Doom of Eztli, Heart of the Elders Part 2, The Depths of Yoth
The Doom of Eztli: Basically reads "Add Ancestral Fear to the victory display," adding doom to locations in this scenario causes a lot of problems (, , and become worse, Final Mistake gets more difficult).
The Doom of Eztli Part 2 Heart of the Elders Part 2: Just like in The Doom of Eztli, you'll usually want to add this to the victory display to avoid losing too many turns (remember, doom on locations sticks around when The Lonely Caverns advances).
The Depths of Yoth: Here, the first "choice" actually becomes a viable option, as you can clear doom by advancing to the next depth level. In addition (for some reason), Forgotten Ruins isn't paired up with Deadly Traps in this scenario, so there isn't as much punishment for ending up with doom on locations.
Strategy: In standalone, Ancestral Fear becomes very interesting, forcing you to be very thoughtful about what other cards with Vengeance you add to the victory display lest you cross the 3 Vengeance threshold for Serpent from Yoth and The Depths of Yoth's . In campaign mode... I'm not going to lie, I hate this thing's design when it comes to campaign mode. Vengeance, ideally, should provide the players a choice a la Offer of Power, forcing them to weigh long-term costs against short-term gains. Boa Constrictor does so by forcing the players to either deal with an annoying hunter or use up 4 damage worth of resources and make things worse later, Serpent of Tenochtitlán tempts the players with that victory point, while Descent to Yoth doubles the temptation and cost. With Ancestral Fear, though, the first "choice" is usually so untenable that it doesn't feel like a choice at all until you reach Depths, making Vengeance less of a choice and more a simple "do bad things" mechanic. And its other keywords (Peril and Surge) mean that your friends can't help you with your "choice" and you'll get another encounter card for your pleasure. If you're playing Campaign Mode, understand that, even if you're a complete pacifist towards all snakes (though not non-snakes, like this jerk), you'll still probably end up with around 4 Vengeance due to Ancestral Fear. If only Yig was willing to talk it out...