The basic idea is this:
DemonAction — Attack
Cost:
[Some moderate bonus, I'm thinking +
]
Each other player receives [some of the set-aside hexes]
—
Setup: Set aside the top n Hexes face up.
This looks similar to
Tormentor. Cool, if you were re-designing Nocturne don't include them both. I don't care that much that it's similar. Demon makes it feasible for the same Hex to be dealt out multiple times in succession, which is different, and when Demon is stacked, it might change which Hex is strongest;
Bad Omens followed by
Famine seems like a pretty mild attack.
If you set aside the top 1 Hexes and it's
Delusion, and the on-play causes each other player to take at least one of the set-aside Hex, I think it's ridiculously powerful. Also, a stackable Demon that always gives out Greed is BANANAS. So it should set aside at least two Hexes, and it can't give out all of them. Which ones should it give out? How should that be determined?
If it's simply "the attacker chooses some subset of a fixed size", we're back at perma-Delusion, so the victim has to make some choices. If both attacker and victim makes choices it might become slow. Let's try out the simplest version of this idea:
DemonAction — Attack
Cost:
+
Each other player receives a set-aside Hex of their choice (leaving it there).
—
Setup: Set aside the top 2 Hexes face up.
If the two hexes are
Plague and
Poverty, this is almost always a
Torturer with a +$2 on the first play, but it stacks differently—and a lot of the power of Torturer is the stacking. In fact, any time Poverty is set aside, Demon is weaker than
Militia: either it's a $5 Militia—okay, more expensive is not
strictly worse, edge cases everywhere—or the other Hex is weaker than the Militia attack in which case it's even weaker.
Provided your opponent chooses correctly. Hm. Put that in the "problems" column.
If one of the hexes is
Misery and the other is a very strong one,
Demon might simply become a terminal
Harem. Oh wait, there's
Twice Miserable, it might become a terminal Silver worth 4 VP for the first copy (if you get to play it twice) and 0 for each subsequent one. That... sounds fine? I'm not the most expert Dominion player so I might mis-evaluate things, but it seems like it might be slightly bland mandatory buy due to the on-play being worth 4 VP. One of them is definitely better than a Duchy. That is,
if Misery is set aside, for which the probability 1 in 6.
Probably the strongest pair is
Delusion/
Envy—that's bound to hurt an engine one way or the other. Luckily, Demon provides some unenviable virtual money. Self-synergy?
— anyways, this combinations seems
really strong, but it screws with everyone evenly and is thus balanced (lol), and in any case this pair only comes up 1 out of every
666 66 Demon games.
Let's think up one that involves attacker choice as well. Two modes I can think of are pie rule and vetoing.
Pie Rule DemonAction — Attack
Cost:
+
Choose a set-aside Hex. Each other player receives that Hex or the two others, their choice (leaving them there).
—
Setup: Set aside the top 3 Hexes face up.
Veto DemonAction — Attack
Cost:
+
Choose a set-aside Hex. Each other player receives a different set-aside Hex of their choice (leaving it there).
—
Setup: Set aside the top 3 Hexes face up.
If they only set aside 2 Hexes, the pie rule becomes victim chooses (which we already did) and the veto becomes attacker chooses (which is bad). So at least 3 Hexes, and that's probably plenty.
With Veto Demon, the attacker vetoes the (situationally) weakest Hex, the victim vetoes the strongest and takes the middle one. With Pie Rule Demon, the attacker chooses as even a split as possible, which if intuition serves is always done by picking the most powerful. So with Pie Rule, the victim takes the middle and weakest Hex. There might be complications, e.g. if the victim takes
Bad Omens followed by
Locusts, it's close to just being a Locusts, but Pie Rule on the face of it looks stronger than Veto. Also, I'm not quite sure how to compare middle-of-3 vs. weakest-of-2.
However, if it's fairly obvious to both attacker and victim which is most powerful on both the first, second and nth play of Demon, once you think about it enough, the more complex choice rules which include attacker choice might slow the game down for very little benefit, even if picking their poison is fun. So, based on thinking about this and never playing with it, the first design looks best, though the other two should be tried out and probably dropped if they aren't noticeably better.