Finger of Death says:
A humanoid killed by this spell rises at the start of your next turn as a zombie that is permanently under your command
If a player is hit by finger of death and dropped to 0hp, do they turn into a zombie immediately? If they subsequently fail death saves and die, do they then turn into a zombie?
I've always seen it ruled that Finger of Death dropping you to 0 hp would turn you into a zombie. However, a recent Q&A tweet by Jeremy Crawford and a subsequent re-reading of the spell has clarified how this spell works.
Question (@DanDanFielding)
For Finger of Death: if the spell brings the target to 0hp and the target subsequently dies due to failed death saves, dies that count as FoD killing the target? Will it rise as a zombie?
Answer (@JeremyECrawford)
A spell kills you if its damage or other effects slay you. If it reduces to you 0 hit points but leaves you alive, it didn't kill you. #DnD
Let's take a closer look at how dropping to 0 hit points works in 5e:
Dropping to 0 hit points: When you drop to 0 hit points, you either die outright or fall unconscious [...]
The rules go on to explain that dropping to zero hit points knocks you unconscious and causes you to start making death saving throws. Dropping to zero only kills you outright if the remaining damage exceeds your maximum hit point total. For example:
[...] a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack, she is reduced to 0 hit points, but 12 damage remains. Because the remaining damage equals her hit point maximum, the cleric dies.
To apply this to our example, let's say a player has a maximum of 30 hit points and currently has 20 hit points left. If that player is hit by a Finger of Death for 62 damage, they'll be dropped to 0 hit points with 42 damage remaining. Since that damage exceeds their hit point total, they will die outright and rise on the caster's next turn as a zombie under the caster's control.
Another common scenario that could lead to Finger of Death turning a player into a zombie would be if that player has already failed two death saving throws before the spell hits them. When they take damage from the spell, that would count as failing a third saving throw, which would kill them outright and raise them as a zombie on the caster's next turn.
However, if the player hit by the spell is not currently dying, and is dropped to 0 hit points without dying outright, the spell does not raise them as a zombie if or when they die. It simply does a very large amount of necrotic damage.
I would say no, because a PC who is dropped to 0 hitpoints is not killed, but unconscious and dying. The Players Handbook takes some pains to distinguish this.
If damage reduces you to 0 hit points and fails to kill you, you fall Unconscious (see Conditions ). This unconsciousness ends if you regain any hit points.
That itself is pretty-clear: If an attack drops you to 0 hitpoints but doesn't exceed your hitpoint maximum, by RAW it fails to kill you.
For example, let's say Mac the Level 5 Fighter (with a Con modifier of +2) is fighting with some Bandits. He's taken a bunch of hits, so he's only at 5 hitpoints out of his 44 maximum.
One Bandit comes up and and attacks him with its Longsword and beats Mac's AC, dealing 1d8 of damage.The Bandit rolls a 3 on the attack roll and adds a +2 mod for Strength, dealing 5 points of damage. This drops Mac to 0 Hitpoints, and he flops unconscious into the dust.
But Mac isn't dead yet. He's bleeding and dying, but he's "only Mostly Dead." This is why his Party's cleric can use Spare the Dying on him after the party defeats the Bandits in the next turn. But he will die if he keeps sitting there. And that's what the Death Saves represent.
When Mac falls to the ground, he is bleeding. He is not being stabbed repeatedly by a longsword every turn, he is only bleeding. As he lies there, some of his cuts may begin to clot, some of his internal bleeding may subside, and his overall bleeding may slow. This is what a Death Save represents. He also may pull himself out of unconsciousness and crawl away. This is what a Natural 20 on the Death Saving Throw (immediately regain one hitpoint) represents.
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
Let's say there's another case, a few miles away. Rosie the Rogue is walking near a mountain when a Stone Giant takes noticed. Rosie is a level 3 Rogue with a Con Mod of +0, meaning she has 18 Hitpoints. The Stone Giant throws a rock, which crashes into her, dealing a whopping 37 points of damage (rolling 4d10+6). This drops Rosie to 0, but the remaining damage is 19 HP, or exceeding Rosie's max hitpoints, meaning the Rock flattens her and kills her instantly.
If Mac dies, it's not a longsword attack that finishes him-off, it's the bleeding it caused. But when Rosie dies, it is the Rock that kills her directly. And, while this may seem like a silly distinction, it gets important when the effects of spells are added to the mix.
You send negative energy coursing through a creature that you can see within range, causing it searing pain. The target must make a Constitution saving throw. It takes 7d8+30 necrotic damage on a failed save, or half as much damage on a successful one.
A humanoid killed by this spell rises at the start of your next turn as a Zombie that is permanently under your Command, following your verbal orders to the best of its ability.
So, let's look at Mac again. Mac, after the Cleric revived him and completing a Long Rest, decides to go bully a high-level Warlock NPC.
The Warlock, not amused at all, casts Finger of Death on Mac, who fails his saving throw and takes 67 (rolling 7d8+30) points of damage. This once again is enough to send him from 44 to 0, but the damage remaining is 23, which doesn't exceed 44. Therefore, Mac does not die; by RAW, the spell fails to kill him outright, but he is now unconscious once again.
Now, Finger of Death, an Instantaneous-Duration Spell, works by arcing negative energy through the target. After the spell is over, the negative energy (the same kind of energy which, by D&D official Lore, animates Undead) is no longer coursing through Mac's body. In fact, it has pretty-much dissipated after draining most of his energy. If Mac were to fail his next 3 saving throws, it wouldn't be the spell that killed him: the magic is over and done with. Instead, it is the fact that he can no longer procure enough life energy to continue to beat his heart or breathe which kills him.
That is why Mac would not come back as a Zombie, because there is no negative energy left from the spell to animate him, that expired several turns ago when the spell's duration (Instantaneous) wore-off.
While it is certainly possible for an NPC to use this on a PC, far and away the most common target of this spell is going to be an NPC. This is simply because NPC's that use this are going to be exceedingly rare. As a 7th level spell, it will require a 13th level caster. As such, an NPC with access to this spell is not going to be a 'random encounter' - they will typically be rare, planned individuals. Whereas PC's with this spell will be using it often, and using it on NPC's. Thus in any given campaign, the spell will more likely, and more frequently, be used on NPCs.
For NPC's, the default assumption is that they will die at zero hp:
Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.
I think it is safe to say, as @Slagmouth suggests that the design of the spell is contemplating its use on creatures that don't get death saving throws.
In the unlikely event that this spell is used on PC's, there would typically be a dramatic reason. It may be one of the rare games in which PvP is allowed. It might be used by a lich, an archmage necromancer, or other BBEG. If so, there is likely to be some dramatic context to its use. The DM has already planned this encounter to be special. In such a case, how it interacts with death saving throws should support the existing dramatic narrative. That might mean you become a zombie if the spell was what dropped you to zero and then you subsequently died from any effect, it might mean that you are animated only if nothing other than the spell damaged you, or it might mean that if the spell did not instantly kill you, you won't be animated - whatever is most appropriate for the narrative the DM is conveying.
