Dominate

The X here is doing scaling work that fixed-cost theft spells never had access to. Control Magic took whatever you pointed it at for a flat price; this instead lets you decide, at instant speed, how much you are willing to pay to steal a given target, and you pay it on the spot rather than committing an enchantment that can be Disenchanted off later. That instant-speed window is the whole pitch: the spell waits until a creature has resolved, dodges the sorcery-speed counterplay an aura invites, and repurposes a threat mid-combat or in response to an attack. The trade is deliberate and steep. You always spend three mana more than the creature's mana value (eight mana to take a five-drop), so this is never a clean tempo play; it is a mana-negative answer that buys its swing with raw investment, which is why it earns its keep only against something expensive enough that the body more than repays the premium. Where an aura grants permanent control through a permanent that can be answered, this grants control once and for good with no string attached to a fragile object, trading the recurring vulnerability of an enchantment for the up-front tax of paying X. That is the design idea: a theft effect priced to the size of what it takes, resolved on your terms, and immune to the artifact-and-enchantment removal that has always made permanent mind control a liability.