Power of Chaos: Kaiba the Revenge (2004) Yu-Gi-Oh
Yes, your opponent had to spend a card and mana to do that, but if a card was printed that said "if this card is in your opening hand, you can reveal it Leyline style to force your opponent to mulligan after he/she is done resolving mulligans," it would see some play simply because of the free wins
The strongest monster in the archetype has only 3,000 ATK Power, which is strong, but there are stronger monsters in Yu-Gi-Oh
4 The Sibsig Ceremony Rare - $0.69 The source of plenty of combos across multiple formats in Magic, The Sibsig Ceremony is a triple black enchantment that drops the generic mana cost of all your creatures by two, making tons of creatures easier to play, if not free in some circumstances
When you start to see the difference it can make, though, youll quickly understand its power