Niblis of Frost

Most prowess creatures reward you for casting spells; this one rewards you for casting the right spells, then taxes the opponent for the privilege. Every instant or sorcery does triple duty: it pumps the body, it advances whatever the spell was actually for, and it pins down a blocker or attacker for two turns running. The tap-down clause is the part carrying the design. Fire it in your own combat and a creature that would have blocked your swollen flyer is suddenly tapped out; fire it in the opponent's end step and their best attacker is stranded through their next untap, closer to a temporary Pacifism than a one-shot tempo swing. Because the trigger keys off casting rather than resolving, it lands even when the spell fizzles or gets countered, so a cheap cantrip is enough to keep an opposing threat frozen while the flyer keeps connecting overhead. The tension lives in the 3/3 flying frame: large enough to close a game in a handful of hits, soft enough that it wants protection, and the protection happens to be the same cheap interaction that feeds both abilities. It is a finisher that asks for a spell-dense deck and then converts that deck's natural texture into repeated, asymmetric tap-downs, clearing a lane for the attack rather than grinding to an empty board.