Doorkeeper

A mill payoff disguised as a wall. The 0/4 body wants to do nothing but sit there, and that is precisely the point: the activated ability scales off creatures with defender, so every blocker you add is also a fraction of a deck you bury. This is the keystone the "Defender mill" archetype was built around, a deck that treats the battlefield like a manufacturing line, stacking walls behind it until a single activation dumps a fifth or sixth of an opponent's library at once. What balances it is the inseparability of the two roles: the wall is both the win condition and the wall, so you cannot race anyone with a 0/4, and the deck has to survive long enough for the math to turn lethal while only making the math lethal by committing enough fragile bodies to feed the activation. It is a deliberate inversion of how aggression usually works in blue, where the threat is tempo or evasion. Here the threat is patience plus arithmetic, the kind of mill engine that does nothing early and ends the game late if it is left alone. The recurring problem for any deck like this is that it asks an opponent to lose to a clock they can see coming and a board they can ignore; the cards that punish it are the ones that pressure life total fast enough to make the slow plan irrelevant.