Monstrosity
Monstrosity is a keyword introduced in Theros, where it was 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. [1]
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.
Examples
Rulings
- A monstrosity ability doesn't do anything if it resolves for a second time.
- If a monstrosity ability is countered somehow, the creature doesn't gain counters or become monstrous, and the monstrosity ability can be used again.