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:
- Both layers of True Polymorph are suppressed simultaneously and the creature is immediately turned back into a medium object.
- 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:
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).
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?