Worldsoul Colossus

The X here does double duty, and that is the whole engineering problem. Convoke lets your existing board pay for the cost, each tapped creature covering or one mana matching its color, so a green creature can pay the and a white creature can pay the while the rest chip at the . The same X then converts directly into +1/+1 counters on the body that resolves. So the creatures you tap are not just discounting a spell: they are sizing the threat. Tap five bodies' worth of convoke and you have effectively recycled their presence into a single large beater while leaving your lands free. That double-counting (board converted to cost, cost converted to stats) is the wrinkle that lifts it above a plain X/X green fatty; the cheaper you make it through convoke, the larger the X you can afford to declare. The printed 0/0 is a real dependency, not a formality. X is what keeps the body alive, so anything that puts the card onto the battlefield without paying its cost (a reanimation spell, a blink, a cheat-into-play effect) sets X to zero and hands you a creature that dies to state-based actions the instant it arrives, absent an outside toughness buff. Even cast normally, declaring X as zero produces the same dead 0/0, so the survival condition is not the casting method but the counters themselves. It drains a flooded board the way a mana sink drains excess lands, except the resource spent is creatures you already control, and the loop between paying wide and hitting tall is the tension the designer has to watch.