My Submission:

Demonologist • $4 • Action
+3 Cards
If you have Dark Pact, +1 Action. Otherwise, discard a card. If you discarded an Action card, you may trash it to take Dark Pact.
Dark Pact • State
At the start of your turn, you may gain a Curse. If you didn't, discard 4 cards, then +1 Card.
Rather than using a once-per-game Event or a landscape that triggers once per game, I decided to use a State that can only be taken, not returned. Thus, my submission is
Demonologist, along with a
Dark Pact, a State with a copy for each player.
Demonologist is a Smithy variant, which starts out with a 1 card discarding penalty. However, if a player is willing to discard and then trash an Action card, it turns into a very powerful double-lab, but at a substantial cost. This is the once-per-game choice that players have to decide if and when to make. Do they take multiple Demonologists first? If so, their deck is loaded up with relatively weak terminal draw cards. On the other hand, after the take Dark Pact they'll need Demonologists to mitigate its penalty. And, of course, they need an Action to trash (and need it when they want to pull the trigger, which might not happen if they buy too many other cards).
Dark Pact imposes a huge penalty, a much more severe version of Torturer (either giving Curses or discards). Even when the Curse pile empties, the player is still impacted, forced to take the strong discarding option. Initially I had it discard 2, but then I realized that those cards could be replaced with a single play of the enhanced Demonologist. I considered discarding 3, but was worried that in a game with both Cursers and a down-to-3 handsize Attack (Militia, Margrave, Ghost Ship), there was a risk of having no cards in hand. The discard 4, +1 Card solves that, since players will never end up with fewer than 1 card, yet it is actually more severe than just discarding 3.