Primal Cocoon

The trap is built into the upkeep clause: this Aura makes a creature bigger only while it sits there doing nothing, and the moment that creature does the one thing a growing creature is supposed to do (attack or block), the engine destroys itself. That self-sacrifice on combat is the whole tension. The counters stay even after the Aura is gone, so the math rewards patience: leave it on a turn or three before committing, and you bank a permanently buffed body for a single green. Rush in early and you have spent a card to make something a 1/1 larger before throwing the Aura away. It is a design that quietly punishes the impatient and the obvious play, asking the pilot to read the board and decide when growth is worth more than aggression. The other wrinkle is that the trigger fires on attacking or blocking, so a counters-on-creature plan that wants to hold the ground and chump or trade gets no grace period either; the Aura wants you to do nothing at all with the creature it is improving, which is a strange and specific kind of restraint to ask of a one-mana green spell. Cheap, slow, and conditional, it belongs to the family of pump enchantments whose real cost is measured in tempo rather than mana.