Nightsky Mimic

The reward condition is the whole tension here: a 2/1 that becomes a 4/4 flier only when you cast a single spell that is both white and black, not merely a spell castable in those two colors. That distinction narrows the trigger to gold and hybrid cards with the white-black identity actually printed on them, a deliberately tight pool that suits the allied-and-enemy hybrid obsessions of its design era. It is a payoff masquerading as a body: cast nothing on theme and you have a fragile two-drop; chain the right spells and it threatens to close the game from the air. The trigger fires on cast, not resolution, so the boost and flying kick in when the ability resolves after you put a qualifying spell on the stack and persist until end of turn whether or not that spell ever resolves. That makes it a tempo creature rather than a standing threat, cashing in on a turn you were already committing your most color-locked card. The Shapeshifter type does no mechanical work beyond flavor; this is no clone or tribal piece but a build-around that asks you to load a deck with white-and-black spells and treat the 4/4 flier as a rider on cards you wanted to cast anyway. Its ceiling is set entirely by how many genuine white-and-black spells exist to feed it, which makes it a test of one narrow deckbuilding axis rather than a generically efficient body.