Ravager Wurm — ruling 1

You may choose neither mode for Ravager Wurm's triggered ability. In this case, the ability is removed from the stack with no effect.

Ravager Wurm — ruling 2

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.

Ravager Wurm — ruling 3

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.

Ravager Wurm — ruling 4

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.

Ravager Wurm — ruling 5

If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.

Ravager Wurm — ruling 6

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.

Ravager Wurm : règles, fonctionnement et interactions (MTG)

Ravager Wurm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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.

How it works : Ravager Wurm

Original English text — Wizards of the Coast / Scryfall

#1January 25, 2019· Wizards of the Coast
You may choose neither mode for Ravager Wurm's triggered ability. In this case, the ability is removed from the stack with no effect.
No other card is named in this ruling.
#2January 25, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#3January 25, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#4January 25, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#5January 25, 2019· Wizards of the Coast
If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.
No other card is named in this ruling.
#6January 25, 2019· Wizards of the Coast
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.
No other card is named in this ruling.