What it does
Monstrosity is an activated ability (written "Monstrosity N" with a cost) found mainly on creatures from the Theros block. When you pay the cost, you put N +1/+1 counters on the creature and it "becomes monstrous." Many of these creatures have a bonus effect that triggers when they become monstrous—like dealing damage or creating tokens.
The key nuance: monstrous is a one-time status, not a renewable resource. Once a creature is monstrous, you can't make it monstrous again, so paying the cost a second time does nothing (you won't get more counters or re-trigger the "when it becomes monstrous" effect). Note the counters stay even if the monstrous status is somehow irrelevant, and removing the creature resets everything—a new copy isn't monstrous.
In Commander, monstrosity gives big mana sinks that scale into the late game, turning flooded hands into board-dominating threats. Stalwart payoffs like Polukranos or Fleecemane Lion reward you for surviving to untap with extra mana.
Iconic cards with Monstrosity
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