Koma, Cosmos Serpent

A threat built around a resource it prints for itself. The passive token generation triggers on every upkeep, yours and each opponent's, which quietly turns Koma into a fountain of 3/3 Serpents that exist to be spent rather than swung with. That is the real engine: the tokens are ammunition for the sacrifice ability, and the ability splits into two modes that cover both halves of a defensive package. Tapping down a permanent and shutting off its activated abilities for the turn functions as repeatable, mana-free control, capable of freezing an attacker, a mana rock, or a key engine piece every turn cycle. The indestructible mode blanks the wraths, the burn, and the fight spells that usually punish a seven-mana investment (though it does nothing against edicts and other sacrifice effects, which is worth knowing before you lean on it as blanket protection), and because it costs only a Serpent rather than mana, it can be held up alongside a full board. The uncounterable clause on the cast is the concession that makes a green-blue behemoth this reliable: the color pair that most wants to durdle also plays the counterspells that would otherwise make a top-end threat feel like a gamble, so the design pays that tax up front. What results is a body whose 6/6 stats are almost incidental. It closes games eventually, but Koma's identity lives in the loop between the upkeep trigger and the sacrifice outlet: a self-refueling control shell wearing a sea-serpent costume.