Veil of Summer

One green mana that turns the entire blue-black interaction suite into a liability for the player holding it. The genius, and the reason it drew a ban in multiple formats, is the compression: this is a counterspell against counterspells (Spells you control can't be countered), a hexproof shield against targeted removal and hand disruption (Thoughtseize and Fatal Push both whiff), and, when the opponent has already tipped their hand by casting a blue or black spell, a full replacement that draws you a card so the cantrip makes it free. Green historically paid for its color-hosing with sorcery speed or a body attached; this asks nothing. It answers a swath of the two most powerful control colors at instant speed for a single pip, and because the drawback is simply "your opponent isn't playing blue or black," it is dead only where those colors are absent. That asymmetry is what made it oppressive: a card meant as a hoser that reads, in the mirror against interaction, like a Time Walk with upside. The design lesson it left behind is how narrow a color-pie window has to be before a one-mana blowout warps a format, and how "hates on two colors" stops being narrow when those two colors define the interactive metagame.