Ravager Wurm
Riot (This creature enters with your choice of a +1/+1 counter or haste.)
When this creature enters, choose up to one —
• This creature fights target creature you don't control.
• Destroy target land with an activated ability that isn't a mana ability.
Illustrated by Svetlin Velinov
This printing
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If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.
You may choose neither mode for Ravager Wurm's triggered ability. In this case, the ability is removed from the stack with no effect.
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 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 the target of Ravager Wurm's first mode isn't a legal target as that ability resolves, or if Ravager Wurm has left the battlefield, neither creature will deal or be dealt damage.
Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities are activated abilities and will have colons in their reminder text. An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate.
