[RPG] If the Power Word Kill spell is cast on a Polymorphed creature, what happens

character-deathdnd-5epolymorphspells

I have read that the polymorph and power word kill spell combo can be annoying for DMs but I'm confused on why it works.

In the description for the polymorph spell it states:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

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.

When a polymorphed creature dies from power word kill, shouldn't it revert back to its normal form with its previous number of hit points?

How does the power word kill spell affect a creature with polymorph cast on it (whose new form has less than 100 HP)?

Best Answer

It is an insta-kill that requires a 9th-level slot and the enemy to fail a WIS-based save

PWK states that

If the creature you choose has 100 hit points or fewer, it dies.

And Polymorph states that

The transformation lasts for the Duration, or until the target drops to 0 Hit Points or dies.

Given that, from the SRD,

A creature that has died can't regain hit points until magic such as the revivify spell has restored it to life.

Then the creature returns to its original form, but remains dead and unable to gain any hit points.

This may sounds like a broken combo, but it will spend your valuable 9th-level slot and take some turns while you burn the target's legendary resistances in order to work (in the event the target fails the very common WIS saving throw). From experience, decent martial fighters will get a creature within the 100hp range nearly as fast as a group of casters burning the target's resistances, and will do so more efficiently.

This is confirmed in the Sage Advice Compendium:

What happens if I’m polymorphed or Wild Shaped into a creature with fewer than 100 hit points and then I’m targeted by power word kill? You die.