Rooftop Storm

Cost reducers usually shave a generic mana or two off a spell type; this one deletes the price tag entirely. Every Zombie creature spell, from a one-drop to a ten-mana monster, costs nothing, which is less a discount than a license to empty your hand. The trick is that the trade has already happened by the time the enchantment resolves: you spend six mana, in blue, on a do-nothing turn, and the payoff lives entirely in the turns afterward. That front-loading is the whole tension. Against an empty board the card is a beat behind, but if you stabilize and untap with it down, the game stops being about mana and starts being about how many Zombies your library can produce. It pairs naturally with effects that turn casting into an engine: a single recurring Zombie body plus a sacrifice loop becomes an arbitrarily long sequence of free casts, since the recast cost the loop usually has to pay is the cost this enchantment erases. The "you may pay " wording matters too: it is an optional replacement for the mana cost, not a separate spell mode. Choosing skips the printed mana, but additional costs still apply, so a kicker or any extra payment riding on the spell is paid on top of the free cast. It is the rare enabler that asks nothing of your mana once it lands and everything of your deckbuilding before the game starts.