Ancient Witch
Action - Reaction - 5
Trash a card from your hand, +2 Cards
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When another player plays an Attack card, set Ancient Witch aside. You are unaffected by the attack. On your next turn trash this Ancient Witch, if you do, all other players must reveal a Curse in their hands, or gain a Curse (this is considered an attack).
I agree that its a bit clunky, but I think this seems like at least a plausible idea.
One note though, is I see a lot of little notes on fan cards like (this is considered an attack) or (this is not an attack). If you ever find yourself writing something like this, its almost guaranteed to be a mistake. Take a close look at Moat and Lighthouse. They don't say "you are unnaffected by attacks". They very explicitly say "when another player plays an attack card". This has 3 parts to it. First of all, it has to be another player. Second, it has to be an attack card. As in, the card has the word "Attack" as a type. Finally, and most relevant to your example, the card has to be
played. So, even if your Ancient Witch is "considered an attack", it doesn't get
played, so it still doesn't interact with Moat and Lighthouse. As another example, some might wonder why ill-gotten gains isn't an attack. And its because it wouldn't matter if it were. The nasty stuff happens when its gained, and Moat and Lighthouse can't interact with that, no matter what the card type is.
Is there ANY scenario, in which a reaction card punishes the attacker in some way that is acceptable? Probably not, but I'm curious to see if conceptually one could be devised.
Who knows. Maybe? The real question is,
why do you want to make a card that punishes the attacker? The only reason I can think of is that you don't want them to play attacks. But that just seems like bad design. Also, targeting attacks is always going to include stuff like Spy, which is just awful, since Spy isn't really even that mean or powerful.
How about a hybrid victory-reaction? If buying it early has a nasty enough opportunity cost, it might give attacks enough breathing room to still be viable and effective, but then having a big impact on the end game. If someone does really hate attacks and chooses to buy this card early, it still has both a positive (+vp) and negative (deck clogging) impact on them. Anyway, just shooting around some ideas. I'm still not convinced that it would make a
good card, but if its something you really want to make for some reason, it might be possible.
Another idea is to dampen the effect on the attacker. Gain a curse is nasty. But maybe you could have something like "the attacker discards a card or gains a curse", or something like that to give the attacker an out. Again, not sure if that would be a good idea, but I think definitely gaining a curse is almost always going to be too hard to balance, mainly because that's one of the most powerful things an attack can do. So to have a card that counters
any attack with the effect of the strongest attacks is probably going to be no good.