Possessed Aven

Threshold rewrites this card into a different one, and the direction of the change is the whole joke. Before seven cards reach the graveyard, the body is a plain blue flier: a 3/3 with evasion and nothing else to say. Cross the threshold and the creature stops being blue entirely. It turns black, grows, and gains a tap ability that hunts the very color it used to belong to, destroying target blue creatures on command. That self-erasing arc was the design idea this era of black-forward design was built around: several of its blue cards carry punishments for staying blue, color-traitor effects that reward you for filling the yard and then turn on their own kind. The aven embodies the conceit literally, mutating out of its starting color to start killing blue creatures. It is a slow turn (the activation costs plus a tap, and only matters when there are blue targets worth shooting), so the card reads less as a combat threat than as an attrition piece for a patient deck willing to grind a blue mirror once threshold is online. The flavor and the mechanics line up cleanly: a possessed bird, its allegiance flipped, now an instrument aimed at the faction it came from.