Curse of Exhaustion
Most prison effects tax players for their second action; this one simply forbids it. Locking a player to a single spell per turn is a hard cap, not a soft one, and the strategic axis it attacks is volume: combo decks chaining cantrips and rituals, control decks holding up multiple answers, storm-style turns that need to resolve four spells to matter. Against those, one spell per turn is closer to a soft lock than a tax. The catch is targeting. As an Aura that enchants a player, it commits four mana to slowing exactly one opponent while doing nothing to the rest of the table, which is why this kind of single-target lockpiece reads better in duels than in multiplayer, where the unenchanted players keep operating at full speed. It also leans on a quirk of how restriction effects stack: it does not stop activated abilities, mana abilities, or anything that isn't a cast spell, so a player throttled to one spell can still flood the board through tokens already in play, planeswalker loyalty, or equipment. White rarely gets to attack an opponent's resource engine this directly, and the design trades flexibility for that reach: a permanent, color-pie-unusual answer to anyone whose whole plan is doing several things in a turn.
