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?
Best Answer
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:
You can attempt to activate Eldritch Smite in an AMF
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.
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:
Thus, even though it uses a spell slot, there is nothing that says that Eldritch Smite can be considered a spell for any reason.