Lochmere Serpent

A flash 7/7 is the shell; the fuel is the two basic land types a blue-black deck already leans on. The evasion mode sacrifices an Island so the body punches through when it needs to close, and the card-and-life mode sacrifices a Swamp to convert a flooded draw into a slow trickle of resources. Neither is a game-ender on its own; each is a small conversion of manabase into tempo, and each shrinks the land count that made the Serpent castable in the first place. That is the design tension. A controlling deck has to decide, turn by turn, whether it wants a resilient threat or a functioning mana total, because every activation spends the infrastructure that deployed it. The third ability is what makes killing it once accomplish little: for a straight at sorcery speed, it exiles five cards from an opponent's graveyard and returns the Serpent from your own yard to your hand, no land sacrifice attached. There is no cast trigger to strip and no exile clause to fight through; the opponent has to answer a recurring threat that also happens to be incidental graveyard hate. It rewards treating Islands and Swamps as expendable fuel rather than fixed lands, and it punishes greed by eating the mana that gave it its edge. A grinder, not a haymaker, built to be answered twice and still come back for a third pass.