Does killing a creature with a Shadow Blade trigger the Necromancy Wizard's Grim Harvest feature?

Question

School of Necromancy wizards get the Grim Harvest feature at level 2 (PHB p. 118):

At 2nd level, you gain the ability to reap life energy from creatures you kill with your spells. Once per turn when you kill one or more creatures with a spell of 1st level or higher, you regain hit points equal to twice the spell’s level, or three times its level if the spell belongs to the School of Necromancy. You don’t gain this benefit for killing constructs or undead.

The shadow blade spell (XGtE p. 164) states:

You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60).

If I kill a creature with shadow blade, do I regain 4 HP?

Answer 1

No, Shadow Blade deaths don't trigger grim Harvest

The question at hand is "does the spell cause death" when you're trying to trigger Grim Harvest. Short answer: No, the spell creates a weapon. The weapon causes death as a separate activity.

Jeremy Crawford clarifies that "death" is very specific to the source of final damage. Although this is aimed at "Death Savings Throws versus Knocking down to zero HP," it still gives us a nugget to consider:

Q: 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?

A: A spell kills you if its damage or other effects slay you. If it reduces you to 0 hit points but leaves you alive, it didn't kill you.

Emphasis mine, but pay attention to that part. Does Shadow Blade's damage or effects kill the target? Well, the spell doesn't do any damage, it conjures a weapon. So the effect of the spell is to create a nifty weapon. So cast the spell and hold the weapon. Did anyone die? Nope.

Now make an attack with that weapon, via whatever abilities you have at your disposal. The attack roll and subsequent melee attack damage kills the enemy. Although the tool was used, it's not the direct effect of the spell that does the killing.

Consider this: Turn invisible and attack an enemy with 1 HP. Or 0 HP and two failed Savings throws, if it makes it more compelling. You only have one d20 in your bag, but you roll it and get a natural 1. But you have advantage from being invisible, so you roll it again and get a natural 20, hitting and killing the target. You only got to do that because of being invisible. Did being invisible cause the death? Nope. By the same token, creating a Shadow Blade is the effect of the spell. It's the attack with that weapon that causes death, which does not trigger Grim Harvest.

Answer 2

Yes, Shadow Blade deaths do trigger Grim Harvest

TL;DR: Shadow blade's damage could not exist without the casting of shadow blade, and so the spell triggers Grim Harvest.

From MisterB's answer:

Jeremy Crawford clarifies that "death" is very specific to the source of final damage. Although this is aimed at "Death Savings Throws versus Knocking down to zero HP," it still gives us a nugget to consider:

Q: 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?

A: A spell kills you if its damage or other effects slay you. If it reduces you to 0 hit points but leaves you alive, it didn't kill you.

Considering this, there are a few reasons why shadow blade is different from a spell like fireball, but they do not justify not allowing shadow blade to trigger Grim Harvest. I will run through them.

1. The spell's damage comes in multiple increments. Spells that do damage over time or in multiple instances are aplenty. Just to name a few, we have moonbeam, phantasmal killer, and melf's minute meteors. This one is the easiest to speak on. In any instance where this is true but 2 and 3 are not true (e.g. phantasmal killer), the spell is definitively doing the damage, and therefore according to Crawford, should trigger Grim Harvest.
2. The spell conjures something that does damage. A few of these include conjure minor elementals, melf's minute meteors, and evard's black tentacles. A little more difficult, as we have to examine what the "spell's damage" encompasses. There's a grey line here because we could go from something I view as clearly should trigger Grim Harvest, melf's minute meteors, to something that is very questionable, animate dead without many breaks in between.
3. The spell conjures a weapon (or something else that can normally do damage) that does damage. Less common, two of these are: mordenkainen's sword and spiritual weapon.

A rule that addresses all of these differences and also fits inside Crawford's answer is:

If the source could not ever deal damage to the target without you casting the spell, the spell is the one damaging the target.

Dissecting this, this means that spells like animate dead or conjure minor elementals cannot trigger Grim Harvest because the undead or elementals exist beyond your spell, and spells like mordenkainen's sword and spiritual weapon can trigger Grim Harvest because the spectral weapon would not exist without your spell.

Similarly, shadow blade's damage would not exist without you casting the spell.