Zara, Renegade Recruiter

The premise is theft of a very specific kind: not stealing a creature you can see, but reaching into the part of the game an opponent has kept hidden. Most steal effects (Act of Treason, Threaten, the whole line of temporary control-grabs) target the battlefield, where the value is already committed and the opponent has chosen what to expose. This one instead peers at the defending player's hand and pulls a creature that hasn't been cast yet, one still being held for a reason: the bomb they were mana-screwed off of, the reanimation target, the answer they were sandbagging. That the borrowed creature enters tapped and attacking that same player or a planeswalker they control folds the theft directly into a combat you've already declared, turning their own unplayed threat into a weapon aimed back at them for one swing. The design constrains the payoff hard, and the timing is the sharpest constraint of all: the trigger fires only on attack, so you commit to combat before you get to look at anything, wagering an attacking body against a hand you cannot yet see. The creature snaps back to its owner once the turn ends, and it enters attacking rather than free to block or hold up abilities, so this is a tempo raid, not a permanent seizure. What makes the effect sting is that it punishes exactly the disciplined play (holding a threat for the right window) that usually goes rewarded, and it does so mid-combat rather than as a sorcery-speed reset.