Quote from Jshill63 >>

So if you have a magic user that is currently using  a mirror image spell and has 3 mirror images and you cast power word kill and your d20 roll indicates that you targeted a mirror image rather than the actual mage, how does that affect the mage?

 Mirror image only affects attacks. This spell does is not an attack.

It'll only work on creatures whose health points are below 100, so if the creature has more than 100 health points when its used, nothing happens. At least I imagine so, going by the same precedent the Sleep spell goes off of.

“Sticks and stones may break my bones, but words will f***ing kill me.”

Honestly, get something other than this if you're not going against an undead or Undying entity.

How would silence affect this? Would being deaf protect you from it, or would Orcus need to be silenced to be prevented from speaking it?

Both. If it has a max HP below 100 then it will die. If its current HP is below 100 it will also die.

I'm pretty sure it just flat out kills them. Similar to damage that is double the amount of a target's hit points.

Quote from Ulkaur >>

How would silence affect this? Would being deaf protect you from it, or would Orcus need to be silenced to be prevented from speaking it?

 The spell does not specify that the target needs to be able to hear the caster. So silence on the caster would stop it without subtle spell or something similar to remove the verbal component, but silence on the target does nothing to help.

You would Need to have Orcus be silenced to prevent the spell.  It does not matter if the target can hear the word, only that the spell can be cast and its target have 100 HP or less

What happens when a creature has more than 100 hit points???

You revert back when wild shape drops to 0HP or dies. The PHB then goes on to say that when you revert, you go back to your original HP unless excess damage carries over. If that damage sets you to 0HP, then you are unconscious. You aren't dead. 

Quote from Yogue >>

You revert back when wild shape drops to 0HP or dies. The PHB then goes on to say that when you revert, you go back to your original HP unless excess damage carries over. If that damage sets you to 0HP, then you are unconscious. You aren't dead. 

 It doesn't set you to zero hp.  It kills you.  It skips past death saves.  

Quote from MistakenRhyme07 >>

avada kedavra

 That's two words 😉

Give your players a wand or other magic item that can cast this spell at will. However, the power word itself is pre-set, and they don't know what it is. Whenever they say the word in-character, the spell targets whoever they are speaking to, or one random creature who heard it. If nothing else hears the word, the player who spoke it is targeted.

Behold. The most OP spell in all of DnD.

Quote from Hoobie>>

My druid laughs at this. 

1. He casts a 7th level firestorm (concentration)

Bonus action wild shape into a badger and burrows underground

A caster that has access to power word kill almost certainly has access to something that can still harm your badger. Anything that reshapes the ground for example could kill it immediately, leaving your druid trapped underground unable to breathe. Not to mention they can probably just cast dispel magic to end the fire storm.

2.  Chances are, whoever is cating that spell is at disadvantage because of the fire storm all around them.

But after a few rounds I'll emerge from my burrow, then cast MY 9th, True Resurrection on whoever did get hit with PWK,

Power word kill doesn't roll to hit; you choose a target, speak a word and if it has 100 hit-points or less, it dies. You could more easily (and affordably) counter it with silence, counterspell, or death ward.

Quote from NariGimiho22 >>

Behold. The most OP spell in all of DnD.

It's not really overpowered; it's a 9th-level spell that only really does one thing, you could potentially kill someone with wish if your DM is lenient ("I wish my nemesis had never been born"), you also need to know that the target is at 100 hit-points or less or it simply doesn't work, as that's not information players normally have (even if you memories the monster manual, you don't know if your DM is using max hp or not etc., plus your character shouldn't know).

Where it gets really dangerous is when you can combine it with polymorph to turn a target into something that you know has fewer than 100 hit-points then power word kill it, but this is why you give big bads wisdom save protections and/or counterspell; enemies like players have a bunch of ways to counter or mitigate this spell. There's also death ward which counters this automatically and completely (no effect at all) at only 4th-level.

If I use second wind as a bonus action after the spell hits me does that mean I can survive it

You cant use bonus actions on another creature's turn.

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for ppl, who still dont understand the fundamentals: this is a death outright effect. U dont get death saving throws against this, desintegration, or massive dmg. U lose.