Wild Evocation

Every upkeep, the game flips a coin you do not control: it reaches into your hand, picks a card at random, and either drops it on the battlefield (if it's a land) or casts it without paying its cost. The choice you usually own (what to play and when) is the price this enchantment charges, and it charges everyone equally, every turn, forever. That makes it less a value engine than a deckbuilding constraint expressed at the table: the only edge is constructing a deck where any random hit is a card you wanted to play anyway. Lands going straight down is the merciful clause, since a random pull is never a wasted upkeep, just unplanned ramp. The brutal clause is "casts it without paying its mana cost if able," which strips free-spell effects of every guardrail those effects usually carry. The thing you most want cast for free is the same thing your opponent most wants cast for free, and the enchantment does not care whose turn it is, so the fantasy of a no-cost Eldrazi has a mirror image where the spell that ends you arrives at no charge too. It is a free-spell engine built without discipline, and that is the whole bargain: randomness as a tax paid by the whole table, until somebody's free spell finally lands on the right side of it.