Blood Baron of Vizkopa
Protection from white and from black on a white-black creature is the joke at the center of the design: a Vampire that cannot be targeted, blocked, or damaged by the two colors that built it. Targeted removal in its own guild does nothing to it, and combat against the very creatures it shares a color identity with becomes one-sided. That alone would make a 4/4 lifelink body for five mana a frustrating thing to kill in a grindy mirror, where attrition is the whole game.
The transformation clause is the closer bolted onto the survivor. Once you have padded your life total and chipped an opponent down, the 4/4 becomes a flying 10/10 with lifelink, a swing that ends games in one hit while the lifegain races the total even further out of reach. The condition reads steep, but it describes the exact board state a drain-and-grind deck is already building toward: you are at a healthy total because of the lifelink, and the opponent is low because of everything else. The card is designed to reward the deck for doing what it wants to do anyway, then provide the finisher the strategy usually lacks. It sits in the lineage of attrition-proof midrange threats that are easy to play and miserable to answer, the kind of creature whose entire value is that the obvious removal does not apply.
