Is there an upper limit to the damage that Eldritch Smite can deal?

Question

The Eldritch Invocation "Eldritch Smite" says the following:

Eldritch Smite

Prerequisite: 5th level, Pact of the Blade feature

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.

Unlike a paladin's Divine Smite, which says "to a maximum of 5d8", Eldritch Smite says no such thing, so does that mean that a Warlock 5/Sorcerer 15 can use an 8th level spell slot to deal 9d8 damage using Eldritch Smite?

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• Related: What is the damage dealt by Eldritch Smite?

• Related: Can a multiclassed warlock expend a Spellcasting spell slot of a non-warlock class to use the Eldritch Smite invocation? (this was found by MikeQ, and I added it here after asking this; this related Q&A address a core mistake in my question, but I'll leave my question here for others who overlook the same thing I did...)

Answer 1

Eldritch Smite caps at 6d8 force damage, or 12d8 on a critical hit.

The Eldritch Smite invocation specifically requires a "warlock spell slot", referring to spell slots granted by the Pact Magic feature. According to the multiclassing rules, spell slots from Pact Magic do not combine levels with spell slots from Spellcasting.

Spell Slots. You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, and half your levels (rounded down) in the paladin and ranger classes.

Warlock is not listed among the classes here. It gets its own section:

Pact Magic. If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

So a Warlock 5 / Sorcerer 15 would have 3rd level spell slots from the warlock's Pact Magic, and 8th level spell slots from the sorcerer's Spellcasting. They can use their sorcerer spell slots to cast warlock spells they know. However, Eldritch Smite does not involve casting a spell, thus the character cannot expend their sorcerer spell slots for Eldritch Smite.

Suppose the character instead had 9+ levels of Warlock, granting access to 5th level warlock spell slots. If they expend a 5th level warlock spell slot using Eldritch Smite, the force damage is 1d8 + 1d8 per spell level, or 6d8 total. And if the triggering attack was a critical hit, then these dice are doubled for 12d8 damage.