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The forced-block clause is the whole game here. A +7/+7 trample pump is already a serviceable finisher, but the "must be blocked this turn if able" line turns the spell into something closer to removal: cast it on an attacker and the defender is compelled to throw a blocker into a creature that now outsizes nearly anything, then eats the trample overflow on top. The double-green cost and five total mana keep it from being a free tempo play, and the sorcery timing means it commits entirely before blocks are declared, so it cannot ambush an already-set combat the way an instant-speed pump would. That sorcery restriction is what keeps the compulsion fair: the defender sees the buffed body coming and gets to choose which creature to sacrifice, but cannot choose to let it through. It belongs to green's long line of "make my big thing bigger and dare you to deal with it" effects, where the tension is always between raw size and the cost of casting at the wrong moment. The forced-block rider is what elevates it past a plain pump spell: it converts the mere threat of trample damage into a guaranteed exchange, on the defender's worst terms.