Functionally, how does Power Word Kill work?

Question

I was just reading this answer, which introduced me to the Power Word Kill spell:

You utter a word of power that can compel one creature you can see within range to die instantly. If the creature you choose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect (101 hp or more).

Functionally, how does this spell work? If I'm understanding this correctly, a PC can speak a word, and if the target has less than 100hp, they instantly are killed without the chance to save. My reason for asking is that this seems extremely overpowered, to the point where it could be used to kill deities or high-level bosses with little effort (especially those who rely less on having high HP and more on dealing extremely high points of damage - Moon Druids come to mind here).

While I recognize that the DnD system does allow for extremely powerful spells (cough cough wish cough cough), this one in particular seems to stand out.

Thus, functionally, how does Power Word: Kill actually work?

Note: I am not asking whether or not the community subjectively finds this spell overpowered. Rather, I am attempting to confirm how this spell works in practice, in order to decide for myself if it is overpowered.

To add some more specific touch-points to guide a potential answer:

• Are any saves permitted?
• Does the spell need to be heard by the target, or seen being cast?
• How might resistances or type-immunity play a role (if any)?

The purpose of this question is to confirm that this spell works in the way I am imagining, which while to me seems overpowered, if I am incorrect may not be the case.

Answer 1

Power Word Kill works just like it says it does.

The spell's description is complete and exact. If the target has less than 100 hit points, they die. Specifically:

• There is no saving throw.

• The spell doesn't do damage. It just kills the target. Damage resistance or immunity has no effect. Nor does any condition immunity except for spells like Death Ward that specifically guard against such instant death effects.

• There are no special targeting restrictions; the caster only needs to be able to pronounce the verbal component, and see the target in range (60 feet). The target doesn't need to see or hear the caster.

• Since there is no save, magic resistance short of outright immunity to spells doesn't help against this spell. Likewise, Legendary Resistance doesn't help. A counterspell cast at 9th level, or with a successful DC19 spellcasting ability check, will still stop it.

• Usually a spellcaster doesn't know how many hit points a creature has. So there's theoretically some risk of the spell "fizzling".

• It's an enchantment, which means that if the caster has the Split Enchantment class feature from the wizard School of Enchantment, they can affect two targets instead of one.

• Likewise, it's a spell with a single target so a sorcerer with the Twinned Spell metamagic option can affect a second target by spending nine sorcery points.

This is, of course, a 9th level spell, and is appropriately powerful; only a character with 17 levels in Bard, Sorcerer, Warlock or Wizard can learn it. So in terms of balance it needs to be weighed against other capabilities and threats at that level.

Answer 2

"How" it works as a question is a little ambiguous, so since there's already an answer about the mechanics, I'll add one that focuses primarily on the lore so you can decide if the spell is appropriate to your setting or not.

The various "Power Word" spells in D&D come originally from Tolkien. Early D&D was heavily influenced by The Lord of the Rings (to the point where there were lawsuits.)

In Tolkien's universe, the world was sung into being using a particular language. The words of the language are so powerful that uttering them causes immediate effect upon the fabric of reality itself. Of course, uttering the Words of Power comes with a cost, so in addition to knowing the word one must be physically and mentally prepared for the backlash. Gandalf used a Word in his battle with the balrog, and as he's describing it to the others after his return his tone makes it quite clear that it was neither a trivial nor a safe thing to do.

In D&D the difficulty and preparation required is represented by all the Power Word spells requiring fairly high level spell slots, and in some editions their equivalents in Dark Speech require additionally an extra feat to have been expended or the speaker suffers bad side effects including insanity or instant death.

"Power Word: Kill" is arguably slightly under-powered as far as 9th level spells go. It affects only one target, and by the time someone in your party can cast it odds are good that most of the stuff you actually consider a threat has enough hitpoints to be unaffected (at least at first.)

Nevertheless, in all D&D editions mechanically it does pretty much the same thing. Out to close range (however the edition defines that) you cast the spell on something living and if that something has fewer than 101 hitpoints and doesn't have some way to completely stop the spell from affecting it, that something instantly ceases to be alive. The caster is literally ordering the universe to kill the creature.