Threaten

Rent a creature for a turn, swing with it, then feed it to Goblin Bombardment or Carrion Feeder before control reverts, so the loan never gets repaid: every borrow-and-discard effect since has been built from this idiom. It reads like a tempo play (steal the biggest blocker, swing for an unexpected chunk of damage), but the untap clause and the haste grant are doing quiet structural work. Untapping means you can grab a body already spent from attacking or paying a cost, and haste means a freshly stolen creature is live the moment it changes hands. The until-end-of-turn duration is the whole catch: control snaps back at cleanup, which is precisely why the card's best homes pair it with a way to make that snap-back irrelevant, anything that converts a borrowed creature into a permanent result. Act of Treason later reprinted the effect at common and made it a Standard fixture, but this is the original red theft sorcery that turned "your creature, my turn" into a recurring design idiom. The rate has always been fair on purpose; the power lives entirely in what you do with the creature before you have to give it back.