Dead, roll a new pc.

i just imagine u use polymorph on a bbeg he fails the save . next round wipe this out 2 turn kill of a bbeg regardless of hp (not counting legendary resistances or immune to polymorph)

I see this as say the magic word and it does 100 hp damage (no dice rolls) to a creature that you can see

I cast DIE

I suppose it works the same way as Disintegrate, it kills the Druid, even if they are wildshaped, they die.

yeah, basically 

You die. But don't necessarily roll a new character.

At that level, Raise Dead and Resurrection spells are not that uncommon. 

why polymorph -> Power Word Kill when you can polymorph -> Divine Word and save a 9th level slot?

Kill as in roll a new.
This spell is way OP.
I, personally, don't allow it 

Quote from JimiG >>

Kill as in roll a new.
This spell is way OP.
I, personally, don't allow it 

 

 If you are throwing 9th level spells around, the players will have Raise Dead, Revivify, Resurrection, it isn't OP. And having it prepared in the *off* chance that the BBEG you are fighting isn't warded against that sort of magic. Never tell players how much health a monster has unless it doesn't matter. If they cast Power Word Kill on a monster that is "Looking rough" and it has more than 100 HP, no effect, the players just burned a 9th level spell. 

If a character is deaf would the spell work on them as they can not hear the somatic component?

Quote from Thir_the_Commoner >>

If a character is deaf would the spell work on them as they can not hear the somatic component?

There is nothing about the mechanics of magic, somatic components, or this spell specifically that requires a target to hear. So yes, the spell would still work on a deaf creature.

The only roll made when you use this.

Wow but brutal

Twin power word kill

Quote from Ozzie >>

Quote from MyzticManor21 >>

 A Chain Warlock casting it through his familiar can kill someone with an invisible sprite while he's in the next room over.

That's not something a chain warlock can do, though.

 voice of the chain master: allow me to introduce myself

Wand of Orcus

This shouldn’t kill a wildshaped druid. “You automatically revert if you fall unconscious, drop to 0 hit points, or die... When you revert to your normal form, you return to the number of hit points you had before you transformed.”

Quote from Samso1 >>

This shouldn’t kill a wildshaped druid. “You automatically revert if you fall unconscious, drop to 0 hit points, or die... When you revert to your normal form, you return to the number of hit points you had before you transformed.”

 You revert to your normal form, but you're still dead. You now have the same number of hit points as on your character sheet, but PWK doesn't care; you're still dead. 

no saving throws