Bloatfly Swarm

Most counter-bearing creatures treat damage as a race between toughness and burn. This one converts getting hit into a payload delivery system. The prevention clause turns every point of would-be combat damage into subtracted +1/+1 counters, and each counter shed drops a rad counter onto every player at the table, its controller included. That inversion is the design: the creature does not so much take damage as spend its own body to irradiate the room, and the more an opponent leans on blockers to trade with it, the faster the shared rad meter climbs. Rad counters are their own grinding clock, milling and pinging each player as they resolve, so a flying beater doubles as a group-wide degeneration engine that opponents accelerate by trading in combat. The catch, and the reason the design stays fair, is that the ability only sees damage. Destroy effects, exile, and -X/-X wraths all kill it cleanly and generate nothing; a well-timed removal spell simply removes a 5/5 flyer with no radioactive tax attached. The five-counter starting load is the fuel, and it depletes only when something reaches through prevention with damage specifically, which means the creature effectively refuses to be burned or blocked to death until it has finished converting itself into ambient fallout. It rewards a controller comfortable poisoning their own life total in exchange for dragging the whole table down together.