Can a warlock use Eldritch Smite in an antimagic field?

Question

The warlock's Eldritch Smite eldritch invocation (Xanathar's Guide to Everything, p. 56) states:

Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller.

Suppose Bob the D&D 5e hexblade warlock is in an antimagic field cast by the evil wizard who wishes to nullify his powers. Bob attempts to use Eldritch Smite on one of the wizard's minions while in the area of the field.

Would the Eldritch Smite fail because of the antimagic field?

If the Eldritch Smite did fail, then what would happen? Would the spell slot be used up? Not used up?

Answer 1

RAW: Eldritch Smite is suppressed but a spell slot is still consumed

Eldritch Smite is a magical ability and is suppressed in an AMF

Eldritch Smite is a magical ability because it is powered by spell slots as is discussed at length in this other Q&A. Thus, Eldritch Smite will not function in an anti-magic field because:

Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere and can't protrude into it.

You can attempt to activate Eldritch Smite in an AMF

Within the sphere, spells can't be cast, summoned creatures disappear, and even magic items become mundane.

However, Eldritch Smite is not a spell¹ and there are no rules saying or even implying that things that use spell slots should be treated like spells.

There is nothing in the spell description for anti-magic field that says you cannot attempt to activate magical abilities in an AMF even though they will be immediately suppressed. Thus, the normal rules for such abilities apply (meaning you can try to activate it).

A spell slot is still used up

Since there are no rules or descriptions in the spell that say that Eldritch Smite cannot attempt to be activated it can be. There are also no rules that modify anything about the spell slots spent by magical abilities in an AMF. Since there is nothing overriding the normal rules, the spell slot is consumed and the effect is suppressed.

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^{1 - An additional point not mentioned in the linked Q&A on what counts as a spell is that use of a spell slot does not mean the ability is a spell. Crawford explicitly says this in this tweet for example:}

^{Divine smite is not a spell, yet it is fueled by spell slots. Because it is not a spell, it is not affected by things like Rage that prevent spellcasting.}

^{Thus, even though it uses a spell slot, there is nothing that says that Eldritch Smite can be considered a spell for any reason.}

Answer 2

Nothing happens, and no spell slots are consumed (unless Bob the Warlock is standing outside the field and trying to attack something inside it)

As explored in this question, we know how to tell if an ability is magical based on some guidelines set out in Sage Advice. In this case, the Eldritch Smite is definitely capital-M Magic because it is fuelled by the use of a spell slot. Therefore, Eldritch Smite won't work in an antimagic field.

As for the use of the spell slot, the general description of the antimagic field clarifies that:

Within the sphere, spells can't be cast, summoned creatures disappear, and even magic items become mundane.

Within the field, "spells can't be cast", which would mean that the slots aren't used up - it's simply impossible to even try to cast a spell. I would judge that, although not explicitly clarified, the same rule should extend to abilities powered by spending spell slots that aren't actually spells themselves - if Bob tried to perform an Eldritch Smite, it just doesn't work and his slots remain unspent.

The description also goes on to say:

Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere and can't protrude into it. A slot expended to cast a suppressed spell is consumed. While an effect is suppressed, it doesn't function, but the time it spends suppressed counts against its duration.

However, this is basically only noting that spells can be cast from outside the sphere into its area, but those spells are suppressed and inactive within the field's area of effect. A caster outside the field still uses up their spell slot when they try to produce an effect inside the field.

The end result is that while Bob stands inside the antimagic field, he can't use Eldritch Smite at all. If Bob is standing just outside the antimagic field, however, and tries to perform an Eldritch Smite on a target in the field, the target is immune to the effect (as a magical effect can't penetrate the field) but Bob is not prevented from trying, so he would waste his spell slot in this case.

Answer 3

It would not work, but you would lose a spell slot

As has been stated in an earlier answer Eldritch Smite is "capital M" Magic and therefore would be affected by the effects of Anitmagic Field.

Looking at the description of Antimagic Field spells cast within the field are suppressed but are cast.

...A slot expended to cast a suppressed spell is consumed

This appears to conflict with an earlier statement in the spell's description stating that

...Within the sphere, spells can’t be cast, summoned creatures disappear, and even magic items become mundane.

Semantically I don't believe the first quote makes sense unless spells are able to be cast within the sphere. I would put the decision as to the interpretation of this difference down to DM's discretion.

For spells with longer a duration, casting from within and then leaving the antimagic field would allow the spell to continue for however long is left of the spells duration. eg A player would be able to cast something like Mage Armour from within the sphere. The effects would be suppressed and a spell slot would be used. As long as concentration was able to be maintained as soon as the player left the field the effects of Mage Armour would come into effect.

I'm not sure what the rules are for players being aware that they are within an Antimagic field. However either way I would rule that because spells are suppressed rather than blocked a spell slot would be expended if Bob the warlock attempted to cast Eldritch Smite.