JUDGEMENT DAY
It's hard to design a card that steals. I mean, look at all the attempts Donald X has made - most are flops! With this in mind, judging will be somewhat harsh this week.
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Dark Knight by Holger
I like the straightforward stealing, but this is too devastating in its current form. It's good that one can no longer steal oppononent's Dark Knights, but you can still trash them which causes swinginess. Still, handing out +Coffers means Dark Knight is sometimes a trap, and that makes it interesting.
Extortionist by silverspawn
I'm assuming this version still gains the trashed card. The fact that it costs $5 and only attacks $3 and under is cool. This makes it not too savage, as in most games you can play with more expensive cards and avoid the attack, and also you can steal your cards back by buying your own Extortionists. The only downside is that if nobody has cheap cards, nobody will buy this attack at all, and that may happen fairly often.
Ambush/Purse by gambit05
Purse is a sweet design, and I like the idea of gaining and attacking the purses of others, but it's too swingy in the current form. If someone kills your Purse early, you're losing $2 and a trash that turn. That's too devastating for something out of your control.
Thieves Den/Burglarized by anordinaryman
It's nice how this card triggers only when others buy VP. It avoids the problems similar designs have and prevents the game from getting in weird states. But I'm not sure I understand how the card works in multiplayer. If multiple people have Thieves Den, who gains the card that gets trashed? FWIW I think this card could work as a duration attack.
Ambush by X-tra
This is a funky, funky card. Like gambit05's Purse, it has the problem of being swingy, as trashing good cards out of hand can be really, really devastating. But you can defend against this attack somewhat with your own Ambushes, as trashing them betters your hand. Overall, I like it. Here's a variant to consider btw: "each other plays an Attack from their hand, trashing it (or reveals they can't)." and then drop the bottom text.
Lupin by majiponi
It's really cool how this gives out Coffers, and it's certainly a better design than Thief or PShip. But like Thief and PShip, it has the issue you don't want to trash opponent's Coppers. That's probably fine, but man, if you hit Copper you're helping your opopnent more than yourself.
The Beast by Jupaoqq
Cool design. I love the concept of a one-shot stealer, and think pairing it with +3 cards or +$3 works well. Sounds like fun in multiplayer. In 2P though, it's a quite expensive, as losing a $6 card to gain a $4 card just isn't worth it most of the time.
Corrupt Official by Xen3k
Another cool one-shot design, but the first sentence helps your opponents way too much - they can use it to Exile bad cards early, and VP cards later on. The attack only hurts if they have 5 truly good cards in hand, which is rare, and for that reason it's desperately in need of a buff. I do quite like the concept though.
Clutter/Cleaning by mathdude
Clutter obviously compares to Talisman. Talisman is already weak, and I think Talisman has a better wording, but I like how Clutter cost $3 and can gain victory cards too. Cleaning is interesting. If I understand correctly, it gains a single card of N price, and not N cards costing up to $3. This means you can gain Provinces and such, which is cool. The attack is reasonably balanced, but man, everything just insists on there being lots of good $3 and under cards, and I don't see enough games having those.
Highjacker by Timinou
This design gets trashing from opponent's hand right. It does help your opponent a lot, but the huge amount of coin you get is pretty sweet. It's not at all obvious when you want to go for this or not, which is fantastic. Overall, a nice and innovative design that I would enjoy playing with.
Spoiler Heir by fika monster
It's quite terrible to trash opponent's Estates, but there's a slight consolation prize in getting to Exile them. Overall, this is probably too weak in that you really don't want to trash the bad cards of opponents.
Tyrant by NoMoreFun
Cool design. I suspect most of the time players will top-deck bad cards instead of trash them, which really slows the game down to a slog. I so want to love this, but the total, utter slogginess makes me fearful.
Masterwork by spineflu
Love the reverse stealing, but +$4 is a crazy amount of coin for a treasure - too much really and too dominant. In every game you'd want these, as 2 copies gives enough coin to buy Province. I suspect the card would be more interesting if it didn't give such crazy amounts of money, as otherwise it's a guaranteed buy.
Wrangler by Aquila
I absolutely adore how this hands out curses and horses at the same time. That's really clever and a nice way to make a weaker-than-witch junker. As per negatives, the stealing part feels a bit tacked-on, and it's confusing to see +2 Cards on a card that gives horses as you can't play any that you drew. Some lovely ideas here, just wish they were packaged differently.
Carnival by Shael
It's nice to see a masquerade-like effect. Unfortunately, this one's too bonkers when discard attacks are involved. If you make opponents discard down to 3, then play this, you're ruining their hand while also stealing good cards. It's also not clear how this behaves when they moat it. But if those two issues were resolved, I'd quite enjoy playing with this card.
Siege by mxdata
It's good to see some Saboteur-like cards! This one's definitely an improvement of the original Saboteur, and hitting $3 to $4 seems like the ideal price. The debt price is kinda clever, but creates an advantage to open 3/4 vs 4/3, and really I think having it cost $5 would be enough. Cost reduction just isn't common enough for the debt cost to be justified. If anything I'd be more worried about the swinginess of stealing villages. If one player gets all the villages, it's very hard to come back from that.
Beguiler/Hoodwinked by Mahowrath
This one's really cool and clever. It's usually only a weak attack - which is a good - and gives interesting decisions to all players in the games. The card has some oddities in multiplayer, which is an issue, but not a very big one. Overall a sweet design.
Goblin Workshop by grep
I love how it gains cards with the same name as those in the trash. That's awesome. The attack side is fairly balanced too. My only complaint is that it can trash copies of itself. Sound frustrating!
Magic Wardrobe by emtzalex
This one takes some building to use. Either you need two copies of Magic Wardrobe, or you need something like Scrying Pool. I like that about it, as it keeps the relatively strong stealing effect from becoming bonkers. Overall, good balance. The big flaw with this card though is that it's political. If you know the top card of each player's deck, you get to decide which one to target. That goes against my advice in the OP to not allow stealing from 1 player only.
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Winner: Highjacker by Timinou
Runner-up: Beguiler/Hoodwinked by Mahowrath