The thing I am thinking about is Earthquake
But more importantly, it prevents damage to your attacking creatures
With a solid three strength and three willpower that will leave most characters banished or at least smarting, being able to instantly challenge can be a hugely effective way of shutting down a troublesome character on the other side of the table - especially if your opponent thinks theyll have at least a turn to respond to any characters you play before they have a chance to attack
5 Riptide Gearhulk Borderless - $83.65 A bit of a bomb that will no doubt make a huge splash in decks that love to blink cards, Riptide Gearhulk has a ton of potential to cause problems for your opponents
Delirium is a grindy, yet powerful mechanic that takes some set up, but can reap you some sweet rewards
Even something signed by Martin Shkreli, once the most hated man in America, could be valuable