Phantom Wurm — ruling 1

If this creature would be dealt damage from multiple sources at the same time (say, because it's blocked by multiple creatures), all of the damage is prevented and only one +1/+1 counter is removed.

Phantom Wurm — ruling 2

If damage that would be dealt to this creature can't be prevented, the damage is dealt and a +1/+1 counter is removed from this creature.

Phantom Wurm — ruling 3

If this creature has no +1/+1 counters on it (but is still on the battlefield because something else is raising its toughness), damage that would be dealt to it is still prevented.

Phantom Wurm : règles, fonctionnement et interactions (MTG)

Phantom 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

This creature enters with four +1/+1 counters on it. If damage would be dealt to this creature, prevent that damage. Remove a +1/+1 counter from this creature.

How it works : Phantom Wurm

Original English text — Wizards of the Coast / Scryfall

#1December 8, 2022· Wizards of the Coast
If this creature would be dealt damage from multiple sources at the same time (say, because it's blocked by multiple creatures), all of the damage is prevented and only one +1/+1 counter is removed.
No other card is named in this ruling.
#2December 8, 2022· Wizards of the Coast
If damage that would be dealt to this creature can't be prevented, the damage is dealt and a +1/+1 counter is removed from this creature.
No other card is named in this ruling.
#3December 8, 2022· Wizards of the Coast
If this creature has no +1/+1 counters on it (but is still on the battlefield because something else is raising its toughness), damage that would be dealt to it is still prevented.
No other card is named in this ruling.