Riot

Riot is a keyword ability, which allows you to choose between gaining a +1/+1 counter or haste when a creature enters the battlefield. It was introduced in Ravnica Allegiance for the Gruul Clans of Ravnica.

Rulings

 * Riot is a replacement effect. Players can't respond to your choice of +1/+1 counter or haste, and they can't take actions while the creature is on the battlefield without one or the other.
 * If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.
 * If you choose for the creature to gain haste, it gains haste indefinitely. It won't lose it as the turn ends or as another player gains control of it.
 * If a creature enters the battlefield with two instances of riot, you may choose to have it get two +1/+1 counters, one +1/+1 counter and haste, or two instances of haste. Multiple instances of haste on the same creature are redundant, but it allows Gruul to send a strong message to its opponents.

Cards that grant riot

 * Domri, Chaos Bringer
 * Rhythm of the Wild