Blind with Anger

The genre standard for steal-and-swing is locked to sorcery speed, which means a Threaten effect always commits you to your own turn and telegraphs the theft a full attack step ahead. Pushing the same package to instant speed rewrites that math entirely. You can hold this up through an opponent's combat to seize a freshly tapped attacker and turn it on its master, or grab a borrowed blocker on their turn when nobody saw it coming. The untap-and-haste machinery is identical to every Threaten that came before (a creature changing control always counts as summoning-sick for its new controller, so haste is mandatory baggage, not a bonus, and the untap merely readies a body that already swung or paid for something), so the rate was never the point: the speed is. That speed is what the extra mana buys. Four mana sits a full point above the three-mana baseline for this kind of effect, and that point is the premium for holding the steal in reserve rather than broadcasting it. The nonlegendary clause is the second tax, fencing the spell off from the exact body a control-theft effect most wants to point at: the legendary finisher. As an Arcane instant it accepts Splice onto Arcane riders revealed from your hand, so a steal-and-swing turn can fold in an extra spirit or a burn payoff alongside the theft.