The Shapechanger trait is affected by antimagic field
How to determine if something is magical
If we go through the questions in the Sage Advice companion: "Is the breath weapon of a dragon magical?" There are a few questions te determine if something is magical or not:
- 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.
Sage advice: Breath weapon
Cold Breath (Recharge 5-6): The dragon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8) cold damage on a failed save, or half as much damage on a successful one.
Sage advice:
Let’s look at a white dragon’s Cold Breath and ask our
- selves 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 no
- where in its description.
Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons
are amazing, supernatural beings.
Our case: Shapechanger trait
Shapechanger.
The werewolf can use its action to polymorph into a wolf-humanoid hybrid or into a wolf, or back into its true form, which is humanoid. Its Statistics, other than its AC, are the same in each form. Any Equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Now let's look at Polymorph:(emphasis mine)
Polymorph
This spell transforms a creature that you can see within
range into a new form. An unwilling creature must make a Wisdom saving
throw to avoid the effect. A shapechanger automatically succeeds on
this saving throw.
The transformation lasts for the duration, or until the target drops
to 0 hit points or dies. The new form can be any beast whose challenge
rating is equal to or less than the target’s (or the target’s level,
if it doesn’t have a challenge rating). The target’s game statistics,
including mental ability scores, are replaced by the statistics of the
chosen beast. It retains its alignment and personality.
The target assumes the hit points of its new form. When it reverts to
its normal form, the creature returns to the number of hit points it
had before it transformed. If it reverts as a result of dropping to 0
hit points, any excess damage carries over to its normal form. As long
as the excess damage doesn’t reduce the creature’s normal form to 0
hit points, it isn’t knocked unconscious.
The creature is limited in the actions it can perform by the nature of
its new form, and it can’t speak, cast spells, or take any other
action that requires hands or speech.
The target’s gear melds into the new form. The creature can’t
activate, use, wield, or otherwise benefit from any of its equipment.
This spell can’t affect a target that has 0 hit points. As you can
see, it lists both a spell (polymorph) and the effects of an spell
(polymorph).
Conclusion
The 2nd question of the sage advice answers yes, and thus is considered magical. The shapechanger even has an specifically notation in the polymorph spell.
With this information we can conclude that the Shapechanger trait is magical and affected by an antimagic field, the werewolf would revert back to humanoid when in a antimagic field.
Personal Opinion
Because the rules are unclear, it can be interperted in different ways.
Personally I would rule it as supernatural, since the cause of the transformation is lycantrophy (werewolf disease).
But as in all D&D games, the DM can decide the interpertation of the rules.
