Monstrosity is an activated ability, which means you can use it at any time you could cast an instant.

Can You Use Monstrosity at Instant Speed? If a monstrous creature leaves the battlefield then it’s no longer monstrous. It’s nothing more than a marker to say that the creature has used its monstrosity ability already. Monstrosity is an activated ability (it has the structure of :) while monstrous isn’t an ability at all. Is Monstrosity an Activated or Triggered Ability? What About Monstrous?

“Monstrosity” is the ability itself while “monstrous” is the state it becomes after having used monstrosity. We’ve only seen a handful of reprints of this mechanic since the Theros block and it’s possible we’ll never see it printed again given the other options available. There’s always discussion about evergreen keywords and monstrosity is not one of them. One monstrosity cards has been printed since adapt was introduced ( Clay Golem in the Forgotten Realms Commander decks), indicating that it could still see a return in the future despite indications that it’s obsolete. The Simic ( ) mechanic from Ravnica Allegiance, adapt, bears a great deal of resemblance to monstrosity in that it’s another way to add counters to a creature that can’t be repeated, but removing the counters allows you to repeat the ability. Despite returning to Theros back in early 2020, monstrosity didn’t make a return. Since then there have been eight more monstrosity cards printed from a variety of sets like Conspiracy, Commander decks, and Modern Horizons. It first appeared on sixteen cards in Theros back in 2013 and another six in Journey into Nyx, the final set in the block. Like I said already, monstrosity was the Theros mechanic to represent the monsters of Greek mythology. Now that it’s monstrous you can’t use the ability again. It then becomes monstrous and triggers its final ability to destroy a flying creature. This means you can pay six mana to put three +1/+1 counters on Colossus. Let’s take a look at Arbor Colossus as an example. Some monstrosity creatures also have extra abilities that trigger when they become monstrous or are active for as long as they are. The creature then becomes “monstrous” and can’t use the ability again. Monstrosity is an activated ability that lets you pay a cost to put a number of +1/+1 counters on your creature. Fleetfeather Cockatrice | Illustration by Matt Stewart

Monstrosity by Jll