Powers and effects that target 'the villain deck' in Vengeance-style

You choose only one villain deck for anything that targets multiples.  It does make Turmoil, Mistbound, and Takedown less useful, but they were crazy powerful before.

Actually, I think Take Down specifies that "villain cards cannot be played", in which case it's so much better than when playing against just one villain ;). Other cards which say to apply the effect to "the villain deck", however, do require you to choose one deck when fighting Vengeance-style. It's the same as if an effect targets something like "the hero with the lowest hp" and the Sentinels fit that description, you can pick which one of the four gets affected (as opposed to when an effect says "hero target", in which case it is whichever individual Sentinel is applicable).

Not quite. "Hero" in that context is short for "hero character card."

The kind of example you might be looking for is something like "the hero with the most cards in play."

That can't be right - Infection targets a "hero" and you can decide which Sentinel to put it on when it gets played on them. If "hero" means "hero character card" that doesn't work for the Sentinels - they've always been one hero with four character cards...

It depends on context. If it's a "hero" on its own, it means a hero with a deck and you can select any of the Sentinels. If it has a hp modifier, so "hero with the lowest", you'd take the hero character card with the lowest HP, ignoring things like Decoy Projection. If it was "hero target wit the lowest HP" you'd include things like the Decoy, Constructs and Bots.

Hmm, I've never heard of this distinction of "hero" meaning "hero character card". As far as I'm aware, that's never been a thing - there was always only "hero" (of which the Sentinels are one) and "hero target" (of which the Sentinels are four). When did this "hero = character card" thing come up?

The wording occurs on a lot of villains. Bomber Blade's flip side, Citizen Autumn,the Chairman, Fiendish pugilist and then I got bored.

I know it's come up in playtesting in the past; not sure if/when it might have come up in public conversations...

It's as old as the Enhanced Edition Rulebook.

Hero:  A Hero is a character played by a player.  Heroes have Character cards which list the maximum HP of the Hero and their innate power on the front, and their incapacitated abilities on the back.  Each Hero has a deck of 40 hero cards.

So...the thing about being able to choose which Sentinel something applies to if a "hero" (as opposed to "hero target") effect applies to one of them is wrong?

Nope, just a little ambiguous: