disclaimer: I am new to the forum and I have no idea if this has already a topic. Delete it or merge it if it has.
How about a deck of cards that curse you (not others) when you buy them or use them?
They can be exact copies of already existing cards mechanics, only that they will cost 2 coins less when you buy them.
Example: Cursed Village, Action, cost:1, +1 Card, +2 Actions, When you buy this, gain a Curse token.
Or they can cost 3 coins less and curse you every time you use them?
Demonic Contract, Action, cost: 4, Trash a card from your hand. Gain a card costing up to 3 coins more than the trashed card. Gain a Curse token.
The result is a game more dependent on gathering Victories and Actions, and less on Treasures (although there can be cursed Treasure cards you can buy cheeper, like Cursed Gold with a cost of 4).