One of my players has a special crystal that emits an anti-magic zone 5-feet around them. It was just picked up so they're trying to figure out what to do with the crystal. Suddenly they are ambushed by a Green Dragon who opens with its Acid Breath. So the question is, would the crystal's anti-magic zone negate the dragon's breath?
No, an anti-magic zone would not affect dragon's breath
The dragon is a magical creature, but the dragon's breath weapon is not listed as a magical effect. Therefore, an anti-magic zone would not affect it nor nullify it.
Ask yourself these questions about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
If your answer to any of those questions is yes, the feature is magical.
Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.
A dragon's breath weapon is not a magical effect for the purpose of game mechanics.
No, a dragon's breath weapon is not magical
A dragon's breath weapon is not magical, as stated in the Sage Advice Compendium where they make an official ruling on this exact case (bold for emphasis mine):
Is the breath weapon of a dragon magical?
If you cast antimagic field, don armor of invulnerability, or use another feature of the game that protects against magical or nonmagical effects, you might ask yourself, “Will this protect me against a dragon’s breath?” The breath weapon of a typical dragon isn’t considered magical, so antimagic field won’t help you but armor of invulnerability will.
This unofficial tweet from Jeremy Crawford also reiterates this ruling:
The breath weapon of a typical dragon is not magical.
This means the anti-magic zone would not affect the dragon's breath weapon at all.