Blue Sun's Twilight

Control-magic effects have always paid for their permanence in different ways: an aura you can destroy, an enchantment that returns the creature on trigger, a fixed mana value ceiling. Here the currency is X itself, doing double duty. The same variable that sets the price also gates what you're allowed to steal, so the spell scales with your read of the board rather than committing to a target size in advance. Small X snags a mana dork or a hatebear cheaply; large X reaches the finisher, and once you've overpaid past the five threshold, hands you a token copy of it as well. That copy clause is the wrinkle worth dwelling on: it converts an otherwise clean permanent theft into a spell that leaves you two creatures instead of one, and because the token is a genuine copy rather than the original, it stays yours even if the stolen creature is later bounced or returned to its owner. Structurally this is a Mind Control folded into a scaling sorcery, then handed an incentive to spend beyond what the theft alone requires: the point where a control spell stops being about neutralizing a threat and starts being about compounding one into board presence. The design rewards patience without the fragility that hobbled earlier control-magic cards, since there is no aura to blow up and nothing to sacrifice back.