3 a bit more natural while preserving the spoiler
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As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that the spell mimicked by the "voice" doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to the "voice"this effect without substantially rewriting the spellit. You could, if you wanted, rule that "one PC is subjected to the "voice"its effects unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that the "voice" doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to the "voice" without substantially rewriting the spell. You could, if you wanted, rule that "one PC is subjected to the "voice" unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that the spell mimicked by the "voice" doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to this effect without substantially rewriting it. You could, if you wanted, rule that "one PC is subjected to its effects unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

2 I tried my best to make it spoiler friendly since OP tried so hard to not mention the name of the spell.
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As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that PWKthe "voice" doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to PWKthe "voice" without substantially rewriting the spell. You could, if you wanted, rule that "one PC is subjected to power word killthe "voice" unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that PWK doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to PWK without substantially rewriting the spell. You could, if you wanted, rule that "one PC is subjected to power word kill unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that the "voice" doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to the "voice" without substantially rewriting the spell. You could, if you wanted, rule that "one PC is subjected to the "voice" unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.

1
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As you said, the spell doesn't require a saving throw - it's entirely dependent on remaining HP. So I'm pretty sure this is a mistake.

Possibly either someone forgot, while writing the adventure, that PWK doesn't call for a saving throw, or development on this adventure was begun before that detail was finalized, and that slipped through editing. Regardless, there's no logical way to apply a saving throw to PWK without substantially rewriting the spell. You could, if you wanted, rule that "one PC is subjected to power word kill unless they make a DC 23 Constitution save" or something like that, but it's tough to imagine they wouldn't have said that if that's what they meant.