Alien Invasion

A token engine that snowballs on itself, and the mechanic that makes it dangerous is the invasion counter reading its own history. Cast it in your precombat main phase and it triggers that same turn, minting a throwaway 1/1 with haste, forced to attack, easily blocked or ignored. But every combat the enchantment survives raises the floor: the next token arrives as a 2/2, then a 3/3, and a few turns on you are stamping out hasty attackers with several counters stapled on, each a fresh threat that has to be answered the turn it appears. The design trick is that the growth lives on the permanent, not the tokens, so removal aimed at any single Alien does nothing to slow the curve; the only real answer is destroying the enchantment, and the payoff for leaving it alone compounds every turn. That "attacks each combat if able" clause is the honest cost: these tokens will not sit back on defense, so an opponent stabilizing behind blockers gets to chew through the small ones while the counter climbs. It rewards being ahead on board and punishes being behind, which is exactly the shape a red-green threat engine wants. The first token is nearly free; the tenth is a game-ender, and the gap between them is measured in combat steps you were not able to interrupt.