The new art is sick, reminds me of scandinavian trolls!

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

Quote from Jurmondur >>

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.) Tis the question.

I would say the spell is more general. After all, power word kill lets you choose a creature, and a troll is a specific creature.

Fire bolt followed by massive fighter attacks.....

This is the way.

I don't love every monster in this book, but the troll looks like a huge improvement. Love the loathsome limbs feature!

Quote from Cleseus_Of_Castigonia >>

Quote from Jurmondur >>

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.) Tis the question.

It's an interesting question, and one with a concrete answer. Let's examine the trolls Regeneration ability:

Regeneration. The troll regains 15 Hit Points at the start of each of its turns. If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn. The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate.

This is an if-then statement: If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate.

Now, let's look at power word kill:

Power Word Kill: You compel one creature you can see within range to die. If the target has 100 Hit Points or fewer, it dies. Otherwise, it takes 12d12 Psychic damage.

This is also an if-then statement: If the target has 100 Hit Points or fewer, then it dies.

Is there tension between these two statements? Not intrinsically. We can combine these two if-then statements into a single statement (because they both have the same component of "the target dies"): If the target has 100 Hit Points or fewer, then it starts its turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast on the Troll, the Troll starts its next turn with 0 Hit Points and doesn't regenerate.

Thus, both statements are true simultaneously. I hope this helps clarify things!

Quote from TheOtherRyan >>

Quote from Cleseus_Of_Castigonia >>

Quote from Jurmondur >>

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.) Tis the question.

It's an interesting question, and one with a concrete answer. Let's examine the trolls Regeneration ability:

Regeneration. The troll regains 15 Hit Points at the start of each of its turns. If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn. The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate.

This is an if-then statement: If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate.

Now, let's look at power word kill:

Power Word Kill: You compel one creature you can see within range to die. If the target has 100 Hit Points or fewer, it dies. Otherwise, it takes 12d12 Psychic damage.

This is also an if-then statement: If the target has 100 Hit Points or fewer, then it dies.

Is there tension between these two statements? Not intrinsically. We can combine these two if-then statements into a single statement (because they both have the same component of "the target dies"): If the target has 100 Hit Points or fewer, then it starts its turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast on the Troll, the Troll starts its next turn with 0 Hit Points and doesn't regenerate.

Thus, both statements are true simultaneously. I hope this helps clarify things!

 That's very interesting. Because I would say that Regeneration is more of a condition :The troll will only die if the conditions are met: If the troll starts its turn, has 0 HP, and does not regenerate at the start of its turn. 
That would mean it would need to take fire or acid damage before it starts its turn if it is going to die.

Technically the spell would not overrule, because there is nothing in power word kill that wouldn't allow the troll to regenerate. So the troll is immune, unless you cast acid splash on it before or something.

Quote from Cleseus_Of_Castigonia >>

Quote from TheOtherRyan >>

Quote from Cleseus_Of_Castigonia >>

Quote from Jurmondur >>

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.) Tis the question.

It's an interesting question, and one with a concrete answer. Let's examine the trolls Regeneration ability:

Regeneration. The troll regains 15 Hit Points at the start of each of its turns. If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn. The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate.

This is an if-then statement: If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate.

Now, let's look at power word kill:

Power Word Kill: You compel one creature you can see within range to die. If the target has 100 Hit Points or fewer, it dies. Otherwise, it takes 12d12 Psychic damage.

This is also an if-then statement: If the target has 100 Hit Points or fewer, then it dies.

Is there tension between these two statements? Not intrinsically. We can combine these two if-then statements into a single statement (because they both have the same component of "the target dies"): If the target has 100 Hit Points or fewer, then it starts its turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast on the Troll, the Troll starts its next turn with 0 Hit Points and doesn't regenerate.

Thus, both statements are true simultaneously. I hope this helps clarify things!

 That's very interesting. Because I would say that Regeneration is more of a condition :The troll will only die if the conditions are met: If the troll starts its turn, has 0 HP, and does not regenerate at the start of its turn. 
That would mean it would need to take fire or acid damage before it starts its turn if it is going to die.

Technically the spell would not overrule, because there is nothing in power word kill that wouldn't allow the troll to regenerate. So the troll is immune, unless you cast acid splash on it before or something.

What you are describing is the if-then statement which I detailed above: If the troll dies, then it started its turn with 0 Hit Points and did not regenerate.

But you are misattributing the part of Regeneration describing how Regeneration can (not must) be negated as a prerequisite to the final sentence of Regeneration detailing under what circumstances the troll dies. (See Troll, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn."). However, Regeneration explicitly states the conditions upon which the troll dies: the troll must start its turn with 0 Hit Points and not regenerate. (See Troll, "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."). Nothing else must happen for the troll to die—no fire or acid damage, nothing.

This brings us to power word kill. The language in power word kill is absolute: if the troll has 100 Hit Points or fewer, it dies. (See power word kill, "If the target has 100 Hit Points or fewer, it dies.") Thus, when power word kill is cast on the troll, it must die because that is the spell's explicit effect.

So, now we have two absolute statements: (1) "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" and (2) "If the [troll] has 100 Hit Points or fewer, it dies." Both of these statements must be true. The way that happens is if, after power word kill is cast, the troll starts its next turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast, that is what must happen, and that is why power word kill intrinsically stops the troll's ability to regenerate without inflicting Acid or Fire damage.

What would make your interpretation correct would be the following language: "This trait doesn't function on the troll's next turn only if the troll takes Acid or Fire damage." That would create the statement: "If this trait doesn't function, then the troll took Acid or Fire damage."

I love this scenario because it really illustrates the beauty of certain spell interactions. Normally, when power word kill is cast, the target (assuming they have 100 Hit Points or fewer) simply drops dead, but creatures that can regenerate have one last moment of life before they die (for opportunity attacks).

I understand that these sorts of things can be tricky. Let me know if you have any questions!

Quote from TheOtherRyan >>

Quote from Cleseus_Of_Castigonia >>

Quote from TheOtherRyan >>

Quote from Cleseus_Of_Castigonia >>

Quote from Jurmondur >>

"The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."

Trolls are immune to power word kill.

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.) Tis the question.

It's an interesting question, and one with a concrete answer. Let's examine the trolls Regeneration ability:

Regeneration. The troll regains 15 Hit Points at the start of each of its turns. If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn. The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate.

This is an if-then statement: If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate.

Now, let's look at power word kill:

Power Word Kill: You compel one creature you can see within range to die. If the target has 100 Hit Points or fewer, it dies. Otherwise, it takes 12d12 Psychic damage.

This is also an if-then statement: If the target has 100 Hit Points or fewer, then it dies.

Is there tension between these two statements? Not intrinsically. We can combine these two if-then statements into a single statement (because they both have the same component of "the target dies"): If the target has 100 Hit Points or fewer, then it starts its turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast on the Troll, the Troll starts its next turn with 0 Hit Points and doesn't regenerate.

Thus, both statements are true simultaneously. I hope this helps clarify things!

 That's very interesting. Because I would say that Regeneration is more of a condition :The troll will only die if the conditions are met: If the troll starts its turn, has 0 HP, and does not regenerate at the start of its turn. 
That would mean it would need to take fire or acid damage before it starts its turn if it is going to die.

Technically the spell would not overrule, because there is nothing in power word kill that wouldn't allow the troll to regenerate. So the troll is immune, unless you cast acid splash on it before or something.

What you are describing is the if-then statement which I detailed above: If the troll dies, then it started its turn with 0 Hit Points and did not regenerate.

But you are misattributing the part of Regeneration describing how Regeneration can (not must) be negated as a prerequisite to the final sentence of Regeneration detailing under what circumstances the troll dies. (See Troll, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn."). However, Regeneration explicitly states the conditions upon which the troll dies: the troll must start its turn with 0 Hit Points and not regenerate. (See Troll, "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate."). Nothing else must happen for the troll to die—no fire or acid damage, nothing.

This brings us to power word kill. The language in power word kill is absolute: if the troll has 100 Hit Points or fewer, it dies. (See power word kill, "If the target has 100 Hit Points or fewer, it dies.") Thus, when power word kill is cast on the troll, it must die because that is the spell's explicit effect.

So, now we have two absolute statements: (1) "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" and (2) "If the [troll] has 100 Hit Points or fewer, it dies." Both of these statements must be true. The way that happens is if, after power word kill is cast, the troll starts its next turn with 0 Hit Points and doesn't regenerate. So, when power word kill is cast, that is what must happen, and that is why power word kill intrinsically stops the troll's ability to regenerate without inflicting Acid or Fire damage.

What would make your interpretation correct would be the following language: "This trait doesn't function on the troll's next turn only if the troll takes Acid or Fire damage." That would create the statement: "If this trait doesn't function, then the troll took Acid or Fire damage."

I love this scenario because it really illustrates the beauty of certain spell interactions. Normally, when power word kill is cast, the target (assuming they have 100 Hit Points or fewer) simply drops dead, but creatures that can regenerate have one last moment of life before they die (for opportunity attacks).

I understand that these sorts of things can be tricky. Let me know if you have any questions!

 I know this is a big quote chain, but I really want to understand this. The troll has a feature that says, and I quote. "The troll dies ONLY if it starts it turn with 0 HP and does not regenerate." The Troll will always regenerate on the start of its turn UNLESS it takes acid or fire damage on the turn before. So it would seem that in order to kill a troll at all, you would need to negate it's regeneration ability.

Power word kill states that the target dies, but the Regeneration feat says that it doesn't die unless the conditions are met. Ergo, it IS a contradiction. So the question is, the very question I asked at the very beginning, Does the spell overrule the feat or vice versa? Which one do we go by.

That's why I am a bit confused about your language, supposing that they both must be true. The words 'Only if' would be unnecessary as their is nothing else that would be able to stop it from regenerating except for acid or fire damage.

 

 I know this is a big quote chain, but I really want to understand this. The troll has a feature that says, and I quote. "The troll dies ONLY if it starts it turn with 0 HP and does not regenerate." The Troll will always regenerate on the start of its turn UNLESS it takes acid or fire damage on the turn before. So it would seem that in order to kill a troll at all, you would need to negate it's regeneration ability.

Power word kill states that the target dies, but the Regeneration feat says that it doesn't die unless the conditions are met. Ergo, it IS a contradiction. So the question is, the very question I asked at the very beginning, Does the spell overrule the feat or vice versa? Which one do we go by.

That's why I am a bit confused about your language, supposing that they both must be true. The words 'Only if' would be unnecessary as their is nothing else that would be able to stop it from regenerating except for acid or fire damage.

I will shorten the chain, and I am happy to explain! These things are tricky and nonobvious.

You are right in thinking that "only if" imposes a condition—a "necessary condition" to be specific. The phrase "only if" in "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" is equivalent to saying "then" because [A: the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate] is true. Thus, the first statement, "A only if B," is the same as saying "If A then B."

For instance, "If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate."

You can also view this statement another way: "If the troll did not start its turn with 0 Hit Points or it regenerated, then it does not die" (the or is needed here because we're negating and). This type of statement is known as the contrapositive (if not B then not A), and it is completely equivalent to the original statement (if A then B).

A key point that I think is catching you up is that Acid or Fire damage are not mandatory for the troll's Regeneration trait to not function. This becomes more apparent if we put the relevant portion of Regeneration into the contrapositive. The trait reads, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn." Let's abstract this like the previous statements: if [A: the troll takes Acid or Fire damage], then [B: this trait doesn't function on the troll's next turn]. So, the contrapositive of this statement is "if not B then not A." Let's reframe this contrapositive using actual language: "If the trait functions on the troll's next turn, then the troll did not take Acid or Fire damage." Here, we negated B to become [B*: this trait does function on the troll's next turn] and we negated A to become [A*: the troll did not take Acid or Fire damage]. Remember, the contrapositive is the same statement as the original statement.

So, we can see that the part of Regeneration describing how Acid and Fire damage disable Regeneration is not saying that Regeneration only ceases to function if the troll took Acid or Fire damage. Rather, it is saying that if the trait does function, then the troll didn't take Acid or Fire damage.

Does this help clarify things?

Quote from TheOtherRyan >>

 

 I know this is a big quote chain, but I really want to understand this. The troll has a feature that says, and I quote. "The troll dies ONLY if it starts it turn with 0 HP and does not regenerate." The Troll will always regenerate on the start of its turn UNLESS it takes acid or fire damage on the turn before. So it would seem that in order to kill a troll at all, you would need to negate it's regeneration ability.

Power word kill states that the target dies, but the Regeneration feat says that it doesn't die unless the conditions are met. Ergo, it IS a contradiction. So the question is, the very question I asked at the very beginning, Does the spell overrule the feat or vice versa? Which one do we go by.

That's why I am a bit confused about your language, supposing that they both must be true. The words 'Only if' would be unnecessary as their is nothing else that would be able to stop it from regenerating except for acid or fire damage.

I will shorten the chain, and I am happy to explain! These things are tricky and nonobvious.

You are right in thinking that "only if" imposes a condition—a "necessary condition" to be specific. The phrase "only if" in "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" is equivalent to saying "then" because [A: the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate] is true. Thus, the first statement, "A only if B," is the same as saying "If A then B."

For instance, "If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate."

You can also view this statement another way: "If the troll did not start its turn with 0 Hit Points or it regenerated, then it does not die" (the or is needed here because we're negating and). This type of statement is known as the contrapositive (if not B then not A), and it is completely equivalent to the original statement (if A then B).

A key point that I think is catching you up is that Acid or Fire damage are not mandatory for the troll's Regeneration trait to not function. This becomes more apparent if we put the relevant portion of Regeneration into the contrapositive. The trait reads, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn." Let's abstract this like the previous statements: if [A: the troll takes Acid or Fire damage], then [B: this trait doesn't function on the troll's next turn]. So, the contrapositive of this statement is "if not B then not A." Let's reframe this contrapositive using actual language: "If the trait functions on the troll's next turn, then the troll did not take Acid or Fire damage." Here, we negated B to become [B*: this trait does function on the troll's next turn] and we negated A to become [A*: the troll did not take Acid or Fire damage]. Remember, the contrapositive is the same statement as the original statement.

So, we can see that the part of Regeneration describing how Acid and Fire damage disable Regeneration is not saying that Regeneration only ceases to function if the troll took Acid or Fire damage. Rather, it is saying that if the trait does function, then the troll didn't take Acid or Fire damage.

Does this help clarify things?

No, this does not help clarifying, at all! This is another level of overcomplication that truly astonishes me. "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" Is the exact same thing as  "[the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate]" You just said the same thing twice and then continued to say "Thus, the first statement, "A only if B," is the same as saying "If A then B." " 

You see the problem?

Contrapositives are completely unnecessary. Just like inverses and converses because all of the different ways are saying the exact same thing. All of it is irrelevant though, because it doesn’t change the fact that feat Regeneration is saying that it only dies if it didn’t regenerate that turn.

Bear with me: If the troll dies, then it started its turn with 0 hp and didn’t regenerate. That was your way of looking at it. Ok. Now if power word kill were to be cast on it. It dies. but if the troll dies that would mean that it started its turn with 0 hp and didn’t regenerate. But it did not. So it’s literally a contradiction between the two feats! However you want to phrase that, it is the exact same case. And so I return back to my original question Again...

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.)

Edit: And I should add. Contrapositives don't even work in all scenarios. For example if the statement is "If it is raining then the ground must be wet" If it is not raining, it doesn't necessarily mean the ground is not wet.

Quote from Cleseus_Of_Castigonia >>

Quote from TheOtherRyan >>

 

 I know this is a big quote chain, but I really want to understand this. The troll has a feature that says, and I quote. "The troll dies ONLY if it starts it turn with 0 HP and does not regenerate." The Troll will always regenerate on the start of its turn UNLESS it takes acid or fire damage on the turn before. So it would seem that in order to kill a troll at all, you would need to negate it's regeneration ability.

Power word kill states that the target dies, but the Regeneration feat says that it doesn't die unless the conditions are met. Ergo, it IS a contradiction. So the question is, the very question I asked at the very beginning, Does the spell overrule the feat or vice versa? Which one do we go by.

That's why I am a bit confused about your language, supposing that they both must be true. The words 'Only if' would be unnecessary as their is nothing else that would be able to stop it from regenerating except for acid or fire damage.

I will shorten the chain, and I am happy to explain! These things are tricky and nonobvious.

You are right in thinking that "only if" imposes a condition—a "necessary condition" to be specific. The phrase "only if" in "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" is equivalent to saying "then" because [A: the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate] is true. Thus, the first statement, "A only if B," is the same as saying "If A then B."

For instance, "If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate."

You can also view this statement another way: "If the troll did not start its turn with 0 Hit Points or it regenerated, then it does not die" (the or is needed here because we're negating and). This type of statement is known as the contrapositive (if not B then not A), and it is completely equivalent to the original statement (if A then B).

A key point that I think is catching you up is that Acid or Fire damage are not mandatory for the troll's Regeneration trait to not function. This becomes more apparent if we put the relevant portion of Regeneration into the contrapositive. The trait reads, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn." Let's abstract this like the previous statements: if [A: the troll takes Acid or Fire damage], then [B: this trait doesn't function on the troll's next turn]. So, the contrapositive of this statement is "if not B then not A." Let's reframe this contrapositive using actual language: "If the trait functions on the troll's next turn, then the troll did not take Acid or Fire damage." Here, we negated B to become [B*: this trait does function on the troll's next turn] and we negated A to become [A*: the troll did not take Acid or Fire damage]. Remember, the contrapositive is the same statement as the original statement.

So, we can see that the part of Regeneration describing how Acid and Fire damage disable Regeneration is not saying that Regeneration only ceases to function if the troll took Acid or Fire damage. Rather, it is saying that if the trait does function, then the troll didn't take Acid or Fire damage.

Does this help clarify things?

 

No, this does not help clarifying, at all! This is another level of overcomplication that truly astonishes me. "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" Is the exact same thing as  "[the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate]" You just said the same thing twice and then continued to say "Thus, the first statement, "A only if B," is the same as saying "If A then B." " 

You see the problem?

Contrapositives are completely unnecessary. Just like inverses and converses because all of the different ways are saying the exact same thing. All of it is irrelevant though, because it doesn’t change the fact that feat Regeneration is saying that it only dies if it didn’t regenerate that turn.

Bear with me: If the troll dies, then it started its turn with 0 hp and didn’t regenerate. That was your way of looking at it. Ok. Now if power word kill were to be cast on it. It dies. but if the troll dies that would mean that it started its turn with 0 hp and didn’t regenerate. But it did not. So it’s literally a contradiction between the two feats! However you want to phrase that, it is the exact same case. And so I return back to my original question Again...

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.)

I'm sorry you feel that way. Allow me to break it down in simpler terms.

Regarding your first point—I explained why "only if" and "then" are interchangeable in response to your comment that treated them as different statements. I cannot tell if you still reject that point or not. Regardless, it is simply true, and you are welcome to consult other sources to confirm as much.

You assert that for Regeneration to not function, the Troll must take Acid or Fire damage, but the rules do not require that. As I explained above, the trait's statement is only that Acid and Fire are two ways of causing Regeneration not to function at the start of the troll's next turn (the explanation of contrapositives was so that you could understand that fact). It does not state that Acid and Fire are the exclusive means of accomplishing that. The bottom line is that power word kill is another means of circumventing Regeneration because it requires the Troll to die.

In truth, you completely ignore my explanation of why power word kill forces the Troll to start its next turn at 0 Hit Points and for its Regeneration ability to not function. I'm not sure what more I can say in that regard.

If you are held up by the timing of the casting of power word kill and the Troll's actual death: don't be. The spell is satisfied because the troll dies. Its statement is true. And this brings us full circle to my initial explanation: the Troll starts its next turn at 0 Hit Points and does not regenerate because that is what is required to satisfy both power word kill and Regeneration.

But if you are still unconvinced, then just fall back on Jeremy Crawford's sage advice for the previous edition: Auto-death mechanics bypass mechanics that rely on dropping to 0 Hit Points. Voilà.

Quote from TheOtherRyan >>

Quote from Cleseus_Of_Castigonia >>

Quote from TheOtherRyan >>

 

 I know this is a big quote chain, but I really want to understand this. The troll has a feature that says, and I quote. "The troll dies ONLY if it starts it turn with 0 HP and does not regenerate." The Troll will always regenerate on the start of its turn UNLESS it takes acid or fire damage on the turn before. So it would seem that in order to kill a troll at all, you would need to negate it's regeneration ability.

Power word kill states that the target dies, but the Regeneration feat says that it doesn't die unless the conditions are met. Ergo, it IS a contradiction. So the question is, the very question I asked at the very beginning, Does the spell overrule the feat or vice versa? Which one do we go by.

That's why I am a bit confused about your language, supposing that they both must be true. The words 'Only if' would be unnecessary as their is nothing else that would be able to stop it from regenerating except for acid or fire damage.

I will shorten the chain, and I am happy to explain! These things are tricky and nonobvious.

You are right in thinking that "only if" imposes a condition—a "necessary condition" to be specific. The phrase "only if" in "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" is equivalent to saying "then" because [A: the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate] is true. Thus, the first statement, "A only if B," is the same as saying "If A then B."

For instance, "If the troll dies, then it started its turn with 0 Hit Points and didn't regenerate."

You can also view this statement another way: "If the troll did not start its turn with 0 Hit Points or it regenerated, then it does not die" (the or is needed here because we're negating and). This type of statement is known as the contrapositive (if not B then not A), and it is completely equivalent to the original statement (if A then B).

A key point that I think is catching you up is that Acid or Fire damage are not mandatory for the troll's Regeneration trait to not function. This becomes more apparent if we put the relevant portion of Regeneration into the contrapositive. The trait reads, "If the troll takes Acid or Fire damage, this trait doesn’t function on the troll’s next turn." Let's abstract this like the previous statements: if [A: the troll takes Acid or Fire damage], then [B: this trait doesn't function on the troll's next turn]. So, the contrapositive of this statement is "if not B then not A." Let's reframe this contrapositive using actual language: "If the trait functions on the troll's next turn, then the troll did not take Acid or Fire damage." Here, we negated B to become [B*: this trait does function on the troll's next turn] and we negated A to become [A*: the troll did not take Acid or Fire damage]. Remember, the contrapositive is the same statement as the original statement.

So, we can see that the part of Regeneration describing how Acid and Fire damage disable Regeneration is not saying that Regeneration only ceases to function if the troll took Acid or Fire damage. Rather, it is saying that if the trait does function, then the troll didn't take Acid or Fire damage.

Does this help clarify things?

 

No, this does not help clarifying, at all! This is another level of overcomplication that truly astonishes me. "The troll dies only if it starts its turn with 0 Hit Points and doesn’t regenerate" Is the exact same thing as  "[the troll dying] can only be true if [B: the troll starts its turn with 0 Hit Points and doesn't regenerate]" You just said the same thing twice and then continued to say "Thus, the first statement, "A only if B," is the same as saying "If A then B." " 

You see the problem?

Contrapositives are completely unnecessary. Just like inverses and converses because all of the different ways are saying the exact same thing. All of it is irrelevant though, because it doesn’t change the fact that feat Regeneration is saying that it only dies if it didn’t regenerate that turn.

Bear with me: If the troll dies, then it started its turn with 0 hp and didn’t regenerate. That was your way of looking at it. Ok. Now if power word kill were to be cast on it. It dies. but if the troll dies that would mean that it started its turn with 0 hp and didn’t regenerate. But it did not. So it’s literally a contradiction between the two feats! However you want to phrase that, it is the exact same case. And so I return back to my original question Again...

which one do we go by: the spells description (that would overrule the feat) or the feat (that would overrule the level nine spell.)

I'm sorry you feel that way. Allow me to break it down in simpler terms.

Regarding your first point—I explained why "only if" and "then" are interchangeable in response to your comment that treated them as different statements. I cannot tell if you still reject that point or not. Regardless, it is simply true, and you are welcome to consult other sources to confirm as much.

You assert that for Regeneration to not function, the Troll must take Acid or Fire damage, but the rules do not require that. As I explained above, the trait's statement is only that Acid and Fire are two ways of causing Regeneration not to function at the start of the troll's next turn (the explanation of contrapositives was so that you could understand that fact). It does not state that Acid and Fire are the exclusive means of accomplishing that. The bottom line is that power word kill is another means of circumventing Regeneration because it requires the Troll to die.

In truth, you completely ignore my explanation of why power word kill forces the Troll to start its next turn at 0 Hit Points and for its Regeneration ability to not function. I'm not sure what more I can say in that regard.

If you are held up by the timing of the casting of power word kill and the Troll's actual death: don't be. The spell is satisfied because the troll dies. Its statement is true. And this brings us full circle to my initial explanation: the Troll starts its next turn at 0 Hit Points and does not regenerate because that is what is required to satisfy both power word kill and Regeneration.

But if you are still unconvinced, then just fall back on Jeremy Crawford's sage advice for the previous edition: Auto-death mechanics bypass mechanics that rely on dropping to 0 Hit Points. Voilà.

There we go. 

Power word kill= Dies

Dies= No regeneration