War Tax

A pillowfort tax that scales rather than caps. Most defensive enchantments of this stripe build a flat wall: an attacker pays a fixed price, or simply can't come in. This one is a dial. The controller chooses X every combat, and because the cost is levied per attacking creature, it asks the aggressor to do arithmetic before declaring attacks: swing wide and the toll multiplies, commit to a single threat and the toll is cheap. The friction lives in that interaction between the chosen X and the width of the assault, which makes the card a soft Propaganda effect that the defending player can price up on demand instead of locking at a single rate. The cleverness is also the limitation. The X is paid out of the defender's own mana, so it competes with everything else the turn wants to do, and the tax resolves before attackers are declared, meaning it discourages an attack rather than punishing one already committed. That puts it in the company of pillowfort pieces whose job is to make combat math unappealing rather than impossible: a deterrent, sustained turn after turn, for the player who would rather not have a wall at all and would rather make every alpha strike cost more than it's worth.