Does Antimagic Field suppress all layers of True Polymorph simultaneously?

Question

Imagine that you have cast True Polymorph to turn a medium object into a Helmed Horror with Spell Immunity to Antimagic Field. If you were to then True Polymorph that Helmed Horror into something else, it would lose its Spell Immunity, as its entire statblock is changed. However, what exactly would happen if the newly True Polymorphed creature walked into an Antimagic Field?

I see two plausible outcomes:

1. Both layers of True Polymorph are suppressed simultaneously and the creature is immediately turned back into a medium object.
2. The most recent layer of True Polymorph is suppressed, at which point the Helmed Horror's Spell Immunity kicks in and prevents the next layer from being suppressed.

I think each of these interpretations has a decent argument:

1. This is how Dispel Magic works. If the Helmed Horror also had Spell Immunity to Dispel Magic, the underlying layer of True Polymorph could still be dispelled, because Dispel Magic reads (emphasis mine):

   For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a successful check, the spell ends.

   Both layers of True Polymorph are on the target, and so both can be dispelled, and there is no reason to suspect they aren't dispelled simultaneously (before the Spell Immunity could ever kick in).

2. Antimagic Field reads (emphasis mine):

   Any active spell or other magical effect on a creature or an object in the sphere is suppressed while the creature or object is in it.

   And while there are two layers of True Polymorph on the creature, it seems reasonable to assume that only the most recent is "active", due to the rules for Combining Magical Effects:

   The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

   This (plausibly, I think) could be interpreted to mean that the most recent layer of True Polymorph is suppressed first, at which point Antimagic Field would suppress the next layer if not for the Spell Immunity.

Is one of these two interpretations unambiguously supported by Rules as Written?

Answer 1

Your assumption stated at the end is correct.

Your second possible interpretation mentions the following:

The most recent layer of True Polymorph is suppressed first, at which point Antimagic Field would suppress the next layer if not for the Spell Immunity.

Once the Helmed Horror is True Polymorphed into another creature, it no longer has Spell Immunity. Its stat block is replaced by that of its new form, and I'm assuming its new form doesn't also have Spell Immunity.

If the True Polymorph is ended on this form and it morphs back into a Helmed Horror, then the Spell Immunity would now be present, and this magical effect is immune to the antimagic field's effect because of the specificity.

The introduction to the game states:

Remember this: If a specific rule contradicts a general rule, the specific rule wins.

Antimagic field has a general effect of "everything in this range", while Helmed Horror's Spell Immunity specifies "ignore this particular thing".

The rule about combination you state means that you have two distinct True Polymorphs. They are not considered one effect, and each is separately active. This is more important for effects that confer a buff. Something currently affected by Shield of Faith to get +2 AC wouldn't benefit from a second casting.

Answer 2

It depends on how you conceptualize "active spells" because the rules don't have an explicit answer

The antimagic field spell states:

[...] Any active spell or other magical effect on a creature or an object in the sphere is suppressed while the creature or object is in it [...]

Here we default to the standard English meaning of "active" (from lexico):

3. (of a thing) working; operative.

We so know from the section on overlapping effects that:

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect [...] from those castings applies while their durations overlap [...]

There are two options:

1. You believe that this makes one spell active and the other inactive:

   Thus, antimagic field removes the active one, and then the Helmed Horror's immunity would prevent it from removing the other.

2. You believe that both castings are active:

   Thus, antimagic field would remove both at once (the spell never states that it goes one-by-one, it simply suppresses all active spells). This means the Helmed Horror's immunity would never kick in.

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Argument in defense of 1.

The existence of "active spells" implies the existence of "inactive spells". Similarly, and suppressed spells are inactive, otherwise, (if we assume suppressed spells are still active) antimagic field would suppress a spell, and then suppress it again (because it's still active), and again, infinitely.

The question is then whether multiple castings of a spell causes one to be suppressed. It's kinda of a mess but post-errata the rules include the following regarding overlapping effects:

[...] only the effects of one of them—the most potent one—apply [...]

And so one of the spells is not currently doing anything, thus it is effectively suppressed and therefore inactive. This means that antimagic field would remove the castings of true polymorph one-by-one.

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Argument in defense of 2.

An antimagic field does infinitely suppress a spell (as it never becomes inactive). This doesn't cause any problems, it's simply something unintuitive.

Alternatively, overlapping spells do not result in one being suppressed or inactive,; the spell is still there and it is still trying to do things. Though this is also true when antimagic field applies, the antimagic field spell explicitly suppresses spells whereas overlapping spells do not.

Thus there is no suppression when spells overlap, and as a result, both spells are active and are cancelled out by antimagic field simultaneously.

Answer 3

The Helmed Horror's Magic Immunity doesn't apply here.

The Helmed Horror has immunity to creator's choice of three spells. Antimagic field is a spell. Thus, it could be declared immune to that spell, and antimagic field (target: self) would have no effect if cast upon it directly (as it could not be caused to have an aura of antimagic field). Antimagic field also would not be able to target it. A summoned Helmed Horror with that immunity wouldn't be caused to wink out, for example.

However, that does not prevent it from targeting or stripping away the polymorph effect. Antimagic Field targets the effect directly, and the effect has no such protection. Note that there is a difference here between how Antimagic Field handles things and how Dispel Magic handles things. Generally, when trying to remove personal buffs, you cast Dispel Magic on the individual - something that Spell Immunity (Dispel Magic) would annul. Antimagic Field is an effect on all magical effects in the area, regardless of what they apply to.

As such, Antimagic Field would (temporarily) remove both polymorph effects.

Answer 4

Yes, everything is suppressed all at once.

The spell says:

Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere and can’t protrude into it.

As it is worded. An antimagic field suppresses any and all effects the second it overlaps their space.

A spell's "active" status and it having an active effect are two different things. The PHB/SRD's section on Casting a Spell states the following under the duration subheading:

Some spells require you to maintain concentration in order to keep their magic active.

Concentration spells lose their active status when they run out of time, the loss of concentration (voluntary by dismissing it or involuntary by failing a concentration check) or the spell being dispelled. Excluding concentration the same rules apply to non-concentration spells. A spell is active as long as its duration is not interrupted.

Antimagic field further states:

While an effect is suppressed, it doesn't function, but the time it spends suppressed counts against its duration.

The duration of spells / enchantments / magic items inside the sphere is not interrupted, as such they are active.

I'll elaborate with the following scenario:

You turn a medium-sized Barrel of Ale into a Helmed Horror (CR4), befriend it for an hour and it willingly lets you turn it into a Warhorse (CR 1/2) so you can ride it into battle.

Going by the rules on _combining magical effects the horror polymorphs into a horse because the second casting is deemed more potent by virtue of being more recent.

You mount your trusty war-casked horror and rush into the fray, barrelling into a creature surrounded by an antimagic field.

The instant the first limb of your mount touches the sphere you'll begin falling, a barrel crashing to the ground beside you.

The supression effect of the antimagic field does not interrupt the duration of the second casting of true polymorph. In essence, the barrel to helmed horror polymorph only gets supressed a second time by the antimagic field.

Due to the way it landed, the barrel rolls on further, exiting the antimagic field.

Once fully out of the field's range, your trusty barrel ultimately returns to the shape of a horse, unless you dismiss the second true polymorph.