By RAW, the Druid dies.
Wild Shape protects you from a few specific things:
When you revert to your normal form, you revert to the number of hit points you had before you transformed (and then take overflow damage).
If you revert and have hitpoints remaining in caster form, you don't fall unconscious as a result of being reduced to 0 hit points.
Power Word Kill bypasses both of these restrictions by killing the Druid rather than dealing damage or reducing its hit points to zero. Returning the Druid to life (or preventing its death) is not one of the things that Wild Shape does.
This is basically the same argument as the one for massive damage, except that there's definitely no sequencing of events. The Druid dies, and Wild Shape doesn't protect you from dying. Game over.
House ruling options are slim here. Killing a druid for using a utility form is harsh. On the other hand, relegating a 9th level spell to "knock a druid out of wildshape once" is also pretty rough.
Does the druids combined HP matter or just the wild-shapes HP?
Just the Wild Shape's. Wild Shape says:
When you transform, you assume the beast's hit point and Hit Dice.
Like most of your other stats, your old hit point total "goes away" while you're in beast form. Your hit points are the beast's hit points.
With that said, adding beast hit points to caster hit points, or using the Druid's base hit points for spells such as Power Word Kill may be a useful house rule.
This ruling is confirmed in the Sage Advice Compendium, the source of official rulings from WotC:
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.
Jeremy Crawford
This has been confirmed by Jeremy Crawford:
If a druid wildshapes into a wolf and is then targeted with power word kill does the druid revert dead or alive?
If you have 100 hp or fewer, power word kill causes you to die. Notice that it doesn't say you drop to 0 hp.
So what is PHB pg. 66 "You automatically revert if.....drop to 0hp, *or Die* ." telling us? Form dies, Druid reverts, yes?
Beast form ends if the druid dies; things like power word kill can end you without reducing hit points.
When reading the action block from left to right it follows this order:
- Target one item. Creature, object, building, whatever they are looking for. Assume creature for this scenario.
- Hit deals 2d8+6 piercing damage and 6d8 radiant damage.
- Conditional if target was hit is a creature. DC 15 con save or die if they have 100 or fewer HP.
So they take the damage then make the constitution save. Proving it is a good idea not to make a Solar mad enough to attack you.
Best Answer
Simplest first, good old counterspell
Counterspell is the first thing that comes to mind and just needs a reaction, a 3rd level spell slot, and a PC to make a 19 DC ability check. Not a guarantee, but gives you a good chance of preventing the spell if it comes down to the wire.
Certain spells and abilities can help buff the party's hit points
Since the spell fails if the target is over 100 hit points, buffing hit points and max hit points can help here a lot. Aid immediately comes to mind to raise your hit point maximum starting at 5 at 2nd level and scaling 5 per level. If you can afford a 5th level spell slot, that's 20 hit points to everyone's max hit points are extended by. In combination with bonus hit points, which can be given with many sources two that come to mind are the Inspiring Leader feat and Heroism, you can easily have enough extra hit points to delay anyone getting to a point where they can be affected.
As a fail safe, bring revivify
If all suggestions fail, there are still ways to bring back a PC. Revivify for a cost can bring them back to life with 1 hit point to at least help minimize the consequences of power word kill.
There are other things that can pair with the above to help like using things like Bardic Inspiration, guidance, enhance ability, or even a divination wizard's portent can make the counterspell pretty much guaranteed. Bringing in resistances/healing along with any resistances like a raging bear barbarian or other effects that give resistances to damage types your hit points will last longer. If revivify itself is not available, it's possible to recreate it with wish.
Nothing above is foolproof, but it should give you enough combinations of tools to get over power word kill. I'd honestly be a bit more worried about the Lich up casting something like cloud kill to 9th level. 9d8 per turn damage even with a save may not have the same oomph as an instant kill, but it being an area of effect a party of 4 can easily be dealt over 100 damage on just the initial casting.