Point of order: It’s unlikely, but a Paladin could survive with 8 hit points in this scenario. Assuming you target the optimal point that catches the Paladin inside the far edge of the cloud, they roll to save once when the spell is first cast and then again at the start of their turn. If the Paladin saves both times (remember they get a bonus to all saving throws after level 6), and all damage die roll ones *both times*, that’s only a total of 5 damage. Two hit points of leeway! Two of the die could even roll twos!  /s

1. Yes, technically he still makes a saving throw. Unconscious creatures automatically fail strength and dexterity saving throws, but there’s no rule that they auto fail constitution (or intelligence, wisdom or charisma) saving throws, and thus should still roll to save at the start of their turn. However, it only matters if the damage has a chance to outright kill him (“If the damage equals or exceeds your hit point maximum, you suffer instant death” while at 0 HP)

2. The results of the constitution save don’t change the result of the automatic death save fail. Doesn’t matter how much damage unless it’s enough to instantly kill, otherwise it’s one automatic fail either way. “If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead.” Only spells with an attack roll (a d20 roll to hit) can crit, so that doesn’t factor in.

3. The spell has a 20 foot radius, not diameter. The cloud is 40 feet across, so they’ll be in the cloud for 4 rounds if it’s positioned optimally and it doesn’t get blown away by a gust of wind spell.

4. According to Jeremy Crawford, each magic missile dart counts as a separate instance of damage, so each one would cause a death save failure, killing the PC instantly. In my opinion, assassinating a PC with no recourse is a pretty crappy thing to do as a DM, but it sounds like you’re dead-set on killing this character.

What happend to you?

Correction: If you cast this on top of an enemy it doesn't count as them "entering the area", so they don't immediately have to make a save, only once it's their turn.

Compare this to the spell insect plague, where it say "when the area appears, each creature..."

Shape a wall of stone or wall of force like a ring, and trap the poison gas inside the ringed wall with a group of enemy creatures.  Won't stop spellcasters that have dimension door or teleport prepared, but wall of stone can't be seen through, meaning simpler forms of teleportation and misty step won't work.   

Yuan-Ti + Poisoner + cloudkill + counterspell (for gust of wind).

FTW!

I want to learn this spell to kill the groundhogs that dig against my foundation. 

Quote from Qoray >>

this + forcecage = op

 just use sickening radiance instead

Quote from BossLogga >>

This would murder so many prairie dogs.

 Lmao

@gabrielrockman they were saying that in response to people who claimed you could use misty step to get to the other side after casting it so that the cloud would stay in one spot. In this case you are starting outside of the sphere so it is at least 40 ft.

This spell is pretty much war crimes.

Seems really good if you're inside or have something burrowing beneath you 

I'm really having trouble finding a practical use for Cloudkill. I mean if I ever want to be guaranteed to kill every regular person in a longer than normal tavern (and I don't feel like using 2 fireballs), yeah I'd drop this mudpuppy down and call it a day. 

As a forever DM, I like it as a villain spell. It’s a battlefield hazard for the party to play around, it’s cinematic as heck, and it does indiscriminate damage to weak commoners that the party might be motivated to protect. It doesn’t hit all at once though, so it isn’t like callously incinerating a cage of orphans. The party has a few rounds to try to break the villain’s concentration as the wave of death bears down on the innocent.

How tall is this?  It seems like the sphere would immediately collapse into a much wider cylinder since the gas is heavy.  I've always said it grows to a 30ft radius / 5 ft height cylinder after the first round so that it follows physics more closely but that's entirely homebrew.

Player: I am at the maw of the dragon but can I see down its throat?

DM: You can see all the way down its dark and damp throat yes. Why?

Player: I cast cloudkill inside of its stomach.

DM: What?

Player: I cast cloudkill inside of the dragon's stomach

DM: Uuuuuhhhh...

Player: Does it work?

DM: What's your spell save DC?

Player: Uuuhh a 15.

*Dice hit the table*

*DM pinches the bridge of his nose*

DM: Okay so the dragon suffocated on cloudkill gas.

*Cheers and laughs around the table as the DM wonders why he even tried to make a dragon the BBEG and expected it not to get cheesed to death*

this is me i ate too much taco bell

Quote from DnDDetective >>

Quote from ArnoLamme >>

It says a creature takes damage when it enters the area for the first time on a turn or starts its turn there. So does that mean a creature takes damage twice in a round if it is pushed in the area or the fog moves into it on the caster's turn? Or is that not what's intended?

 In the scenario you have outlined it would take damage twice.

Caution! This is not accurate. The question was officialy clarified in Sage Advice Compendium and this spell doesn't deal damage when you cast it.

The same with spells with the same timing for their areas of effect: moonbeam, cloud of daggers, spirit guardians and others (see the link for the list of spells)

Update: @Clathorus >> also made a good correction here:

Quote from Clathorus >>

Correction: If you cast this on top of an enemy it doesn't count as them "entering the area", so they don't immediately have to make a save, only once it's their turn.

Compare this to the spell insect plague, where it say "when the area appears, each creature..."

If a party with water breathing is underwater and someone casts it in the water under/around them.
Will they still take damage even though they're completely under?
Will the spell even work under water?

Quote from SuperPranx >>

If a party with water breathing is underwater and someone casts it in the water under/around them.
Will they still take damage even though they're completely under?
Will the spell even work under water?

 Fog is made of tons of tiny droplets suspended in the air, in this case, droplets of poisonous magical liquid.

Technically, this means that this should work underwater because it would create a cloud of poisoned ocean in its range. Just water with magic poison all through it rather than air with magic poison all through it. 

Quote from beefpelican >>

As a forever DM, I like it as a villain spell. It’s a battlefield hazard for the party to play around, it’s cinematic as heck, and it does indiscriminate damage to weak commoners that the party might be motivated to protect. It doesn’t hit all at once though, so it isn’t like callously incinerating a cage of orphans. The party has a few rounds to try to break the villain’s concentration as the wave of death bears down on the innocent.

 Though, if you did have a cage of orphans then this spell would be even more cinematic/nightmarish. The sight of all of them succumbing to the poison would be a reliable way to make sure that your party remembers your campaign forever. RAW, the orphans and commoners are unlikely to last more than 1 turn against it, so unless these commoners all start standing well spaced away from the villain, then your party is still going to see quite a few people dying to the micro-tsunami of poison.