Monstrosity

Monstrosity is a keyword action introduced in Theros, where it is connected to the monster theme of the set. It is defined as giving the creature the ability to become monstrous only once per game. Monstrosity is an activated ability you can activate any time you can pay for it. When it resolves, it checks whether the creature is already monstrous. If it's not, the ability puts a number of +1/+1 counters on the creature and causes the creature to become monstrous.

Monstrous is more like a characteristic of a card. Many creatures with a monstrosity ability have a triggered ability that triggers when the creature becomes monstrous. There are also some cards with monstrosity that gain abilities as long as they are monstrous.

From the ()

Rulings

• Although it is not a verb, it is considered a keyword action in the Comprehensive Rules
• Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
• Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
• An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.

Color percentages

There is a total of 16 cards that involve monstrosity, which divide by color as such:

• Red = 25 %
• Green = 25 %
• Blue = 12 %
• Black = 12 %
• Multicolor = 12 %
• White = 6 %
• Artifact = 6 %