So, there is almost always some level of debate on the “permanency” of True Polymorph. So, I’ve taken the liberty to go through and determine exactly what it takes to end True Polymorph at any point. First, let’s look at its duration, as defined by the spell. 1 Hour, with concentration. Alright, that’s easy enough. The first paragraph also offers us this: “ The spell lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the spell lasts until it is dispelled.”

EDIT: additional statement. Duration in this case, as per the PHB, is "the length of time the spell persists. A duration can be expressed in rounds, minutes, hours, or even years. Some spells specify that their effects last until the spells are dispelled or destroyed." Therefore, if a duration change occurs, the additional qualifiers of "0 hit points or death" still apply as they are not increments of time and thus cannot be defined as a Duration per the Player's Handbook. <End of Edit>

So, the first sentence qualifies our statement: Up to 1 hour, the target drops to 0 hit points, or dies. If concentration is maintained for the full hour, the spell lasts until dispelled, as in the second sentence. How does this interact with the first sentence? From what I can tell, IF concentration is maintained for the full hour, the NEW qualifying statement becomes: “The spell lasts until dispelled, the target drops to 0 hit points, or dies.” This is because the new duration of “until dispelled” only replaces the original duration of 1 hour. In other words, even if True Polymorph is “permanent,” the magical effect never goes away. The target is still affected by True Polymorph, and would ping as being magical as a consequence.

Great, so we have our qualifiers. “But Devan?! What if it *doesn’t* replace the full duration only, and instead ONLY lasts until dispelled?” Good question, hypothetical debater! Let’s go through each of the three uses of Polymorph real quick.

“Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target's (or its level, if the target doesn't have a challenge rating). The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.

The target assumes the hit points of its new form, and 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, unless its new form is capable of such actions.

The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.”

In “Creature into Creature” status, it explicitly states that “When it reverts to its normal form…” as well as “If it reverts as a result of dropping to 0 hit points,” without any qualifying statements indicating that these parts of the rules do not apply if the duration has transformed into “until dispelled,” indicating to me that so far, for Creature into Creature portions of the spell, the revised statement of “The spell lasts until dispelled, the target drops to 0 hit points, or dies.”

“Object into Creature. You can turn an object into any kind of creature, as long as the creature's size is no larger than the object's size and the creature's challenge rating is 9 or lower. The creature is friendly to you and your companions. It acts on each of your turns. You decide what action it takes and how it moves. The GM has the creature's statistics and resolves all of its actions and movement.

If the spell becomes permanent, you no longer control the creature. It might remain friendly to you, depending on how you have treated it.”

Here, the spell specifies “If the spell becomes permanent,” rather than “If the spell lasts until dispelled.” I believe, with all my heart, this is actually a typo due to the fact the spell is still affecting the object as an active effect due to the wording in the first paragraph of the spell. That said, I still see no other clarifying statements indicating any new changes to the spell’s statement of “until dispelled, the target drops to 0 hit points, or dies.” If the Creature (from Object) hits 0 HP, or otherwise dies, it would just turn back into being a normal object.

“Creature into Object. If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form, as long as the object's size is no larger than the creature's size. The creature's statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.”

In here, there are no qualifying adjustments to the duration statement previously. The Object (from Creature) will still retain the effect of True Polymorph upon itself, and if Dispel Magic is used on the object, the spell would end. If the object’s hit points hit 0, the effect ends and the creature is returned to its original form. As far as I can tell in terms of the game, objects cannot die (only be destroyed by any hit points it may possess hitting 0), so for our qualifying statement, it seems to be “The spell ends when dispelled, or the target (object) hits 0 hit points.”

Excellent, great. But what about sending a True Polymorphed creature into an Antimagic Field? Surely that will end the spell? ...Well, it won’t. The relevant wording from Antimagic Field: 

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

“Spells. 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.”

In other words, True Polymorph is still an active spell on the target, and would be suppressed but not removed by an antimagic field. However, if the target within the antimagic field hits 0 hit points, or otherwise dies, the effect of True Polymorph would end should their body be removed from the antimagic field.

The only part that might be up for debate in this regard would be “What if while the target is still in the antimagic field, they are healed to above 0 hit points, and THEN removed from the AMF?” 

Far as I can see, technically that would allow the effect of True Polymorph to remain active. However, outside of perhaps the Healer feat, there are not any non-magical ways to heal a target in the rules of the game, so the effect might as well be considered ended… (But, it is a reason to take the Healer feat if you are intending on cheesing around with a permanent Dragon friend while using True Polymorph perhaps?).

TL;DR When you maintain concentration on True Polymorph for the full hour, the spell remains actively cast on the target, can be detected via Detect Magic due to being an active spell, and will last until it is dispelled (due to "until dispelled" being a specific increment of time as defined by the PHB), the target hits 0 hit points (due to this clause not being replaced, and a separate unrelated clause specifying no effect on creatures with 0 hp), or somehow otherwise dies in that form.

Antimagic Fields do not end the spell, but suppress the effects, and allow a means to potentially kill or end the effect safely by killing the (probably) squishier target inside the Antimagic Field before removing them from it to force the spell to end. The Healer feat MAY be able to heal a target inside the Antimagic Field and thus prevent the spell from ending when the target is removed from the AMF. True Polymorph isn't permanent, as in you cannot ever dispel it or avoid suppressing it, because it remains an active spell with a duration and ending qualifiers.

Additional Edit: Also, another clause of the spell is that it has no effect on a Creature with 0 Hit Points regardless, as pointed out to me in the Discord, that the spell *always* ends on a Creature with 0 hp as per the "This spell has no effect on a shapechanger or a creature with 0 hit points." clause in the spell, so even with the potential argument of "the spell lasts until dispelled period, it says so right there," that second separate clause ends the spell and cause a reversion anyway.

TL;DR the TL;DR: True Polymorph can be defined as ending like this: True Polymorph ends by: Duration (which is time or until dispelled), or Target HP >0, or Target = Dead.

Hopefully final edit: Since people still seem to be disregarding the wording of the spell at least a little bit as far as Shapechanger or creature with no HP not being affected by the spell, here's a sage advice that indicates "Until dispelled, or HP hits 0" to end the spell. https://www.sageadvice.eu/2018/02/26/once-a-true-polymorph-spell-is-made-permanent-does-reducing-to-0hp-still-cause-the-creature-to-revert-to-its-original-form/

"The text of the spell says it has no effect on a creature with 0 hit points. That statement is made after the bit about lasting until dispelled. At 0 hit points? The transformation ends." - Jeremy Crawford

For the first hour the spell will end if dispelled or if the caster reaches 0hp or dies.

After the first hour the spell will end if it is dispelled.

Quote from Brian_Avery >>

For the first hour the spell will end if dispelled or if the caster reaches 0hp or dies.

After the first hour the spell will end if it is dispelled.

 allow me to redirect you to the following clause which is still separate from that first paragraph: "This spell has no effect on a shapechanger or a creature with 0 hit points." also, the second sentence as far as it goes is replacing duration. if it was really meant to only ever last until dispelled (not even accounting for the totally separate clause of "No effect on a creature with 0 hit points"), then why would it only change the duration, and not any of the other ending conditions?

The spell explicitly says that if the concentration is maintained for an hour it lasts until dispelled - with no other qualifying statements.

I would agree with Brian_Avery. The first hour is the hardest part. It drops if the caster loses concentration - it gets dispelled from the target - or the target drops to 0hp or dies. After the hour it only ends if dispelled.

Quote from Emmber >>

The spell explicitly says that if the concentration is maintained for an hour it lasts until dispelled - with no other qualifying statements. I would agree with Brian_Avery - while also adding that of course during the first hour - losing concentration also ends it.

 It also still explicitly states in a separate part of the spell it has no effect on a creature with 0 HP, so it still ends if HP = 0, as stated at the very bottom of the post. also, hi Emm, glad to see I could drag you out from Yahtzee lol

My reading of that sentence is that it's referring only to the initial casting of the spell - especially since that is also where it mentions the saving throw.

Fine. Then here's a sage advice from Jeremy Crawford stating "If the target HP is 0, the spell ends." 

https://www.sageadvice.eu/2018/02/26/once-a-true-polymorph-spell-is-made-permanent-does-reducing-to-0hp-still-cause-the-creature-to-revert-to-its-original-form/

and here is the direct quote for those who don't wanna click the link. "The text of the spell says it has no effect on a creature with 0 hit points. That statement is made after the bit about lasting until dispelled. At 0 hit points? The transformation ends." - Jeremy Crawford

I was just REALLY trying to avoid having to turn to Sage Advice and go by what is actually written in the spell, but y'all forced my hand, lol.

I take all sage advice with a heap of salt - especially as that link also says that Mike Mearls and Jeremy Crawford disagree on that point.

Like I said - the placement of that sentence saying it can't effect a 0hp creature is referring to the initial casting. You can't cast it on a 0hp creature or on an unwilling creature who succeeded on their save. Once it is cast and lasted the hour - it's permanent until dispelled.

Edit: Worth mentioning that this is just the way I read the spell. I don't expect everyone else to share my opinion.

Quote from Emmber >>

I take all sage advice with a heap of salt - especially as that link also says that Mike Mearls and Jeremy Crawford disagree on that point.

Like I said - the placement of that sentence saying it can't effect a 0hp creature is referring to the initial casting. You can't cast it on a 0hp creature or on an unwilling creature who succeeded on their save. Once it is cast and lasted the hour - it's permanent until dispelled.

 Then let's apply the same logic to Hold Person. Hold Person now has a clause saying "If you maintain concentration for the full minute, it lasts until dispelled." but it also says that if they take damage, they can make a save to end the spell.

By your reading, that means that with a 2nd level spell slot, and 1 minute... congratulations, you permanently paralyzed Tiamat or the Tarrasque or <insert high CR monster here> no matter how often you deal them damage and how often they make their save. Does that make *any* sense to you?

If it had that clause it wouldn't be a 2nd level spell any more - it would probably be 5th-9th. It also wouldn't prevent the saves when taking damage because that is explicitly defined in the spell as a way for it to end. And it likely wouldn't be worded that way either. Then again - maybe it would.

The devs aren't very good at wording their spells - which is why these discussions happen in the first place.

And True Polymorph explicitly states it has no effect on a shapechanger or a creature that has 0 HP. This is a totally separate clause from the End Spell clauses.

And if it's criteria are met, as in "I can no longer affect this creature, it is at 0 HP," the only follow up logically is "My effect must end, it is no longer a valid target because it is at 0 HP."

I already gave my opinion on that in reply #6 and #8.

So then does Hold Person no longer work if the paralyzed target is turned into an owlbear? They were a humanoid when the spell was initially cast, but now they are a monstrosity. Does Hold Person keep them paralyzed even though they are no longer a valid target after the initial casting?

I would say yes it continues to work. And a similar question for you would be what if you True Polymorphed someone into a Shapechanger? Would it instantly fail and waste the 9th level spell slot?

Like I say - I'm not expecting you to agree with me. I'm only justifying why I read the spell the way that I do.

As a matter of fact, yes. True Polymorph would go through, the target is a shapechanger... It no longer is a valid target because it has no affect on shapechangers, and the target instantly reverts back.

All concentration spells end if the caster is reduced to 0 hp or dies, that's just how concentration works.

If the target is not valid due to being at 0 hp or a shapechanger then true polymorph doesn't work anyway.

If the target is valid and you cast true polymorph then the targeting and casting parts are over, if you choose to polymorph your valid target into a shapechanger you can, once it becomes a shapechanger it doeen't end the true polymorph spell because true polymorph is no longer targeting, the spell has already been legally cast. Now you have to follow the rules for ending the spell, dispelling or 0 hp or dying, and that is only for the first hour, after that only dispelling.

With the hold person spell they only have to be a valid target for the casting of the spell (this is the case with most spells) and the duration is up to 1 minute. It ends early if the save is made.

Just to be clear, if after the hour is up and the target drops to 0 hp it does not change back... the effect of true polymorph is then permanent except for dispel magic.

So, I'm not sure if this is of any help, but I've always interpreted the "no effect on a creature with 0 hit points" only applying to the initial cast. I read the Sage Advice as well, and I take it with less than a grain of salt. Some of the rulings in that are nonsense. This one is good, but it's really up to DM interpretation. 

Yes, the part about it not effecting a creature with 0 hit points follows the part about its "permanence" but the format doesn't really mean much to me when I read spells. 

I read the phrase "it lasts until dispelled" to be exactly that. The phrase gives one situation where the spell would end, and that's it. 

Not even remotely. Once concentration has been maintained for an hour, the victim is permanently turned into a mundane goldfish with a spell affecting it. If a bigger fish eats it, it then becomes poo. The spell may still end when the goldfish dies, but it stays a dead goldfish, it doesn't revert to its original form. All of the rest of that magical fish/magical fisherman stuff is silly. That's you putting things onto it that the spell does not imply.

The spell ends because the creature died, it just doesn’t revert. It stays a dead goldfish until the pop process happens.