Opal Titan

Park it on an empty board against a control mirror and it does nothing: an enchantment that costs four mana to deal no damage and block no attacker. The opponent decides when it matters. The moment they cast a creature spell, it stands up as a 4/4 Giant with protection from every color in that spell, and the protection is the elegant part. It is reactive coloration, the body tuned to be unblockable by, and immune to removal sharing color with, whatever just prompted it to animate. Against a mono-color creature deck, that means an attacker the opponent's own threats cannot trade with and their colored burn cannot touch. The animation is not yours to time or aim; it is dictated entirely by what the opponent chooses to play, which makes it inert against spells and decks that hold creatures back, and turns it on the instant they flood. That conditional shift, an enchantment that becomes a creature only in response to the opponent's choices, belongs to white's old toolbox for taxing aggression without spending a card on every threat: it answers a creature plan by becoming the thing that plan cannot profitably attack into. The cost is borne entirely by the opponent's tempo, and the reward is a clock they handed you themselves.