What is the most damage one can do with a cantrip in one turn?

Question

I'm curious as to what the maximum damage you could potentially do with a single cantrip is.

So far the most I can think of is a Warlock of at least two levels with the Eldritch Blast cantrip and the agonizing blast invocation and a 20 in CHA with a total character level of 17 or higher for 4 beams against a target afflicted by Hex and Hexblade's Curse. Which is a total of 1d10+1d6+8 per beam for a grand total of 96 maximum damage (4d10+4d6+32). Beyond that however, I can't think of anything.

Stipulations:

• Preferably contain all setup and the casting of the cantrip in the same turn but if necessary you may have one turn to prepare.
• The damage MUST be done by the cantrip but spells like Hex that don't directly deal damage are ok.
• Don't factor in resistance or vulnerability.
• Since we're talking theoretical maximum damage you can assume any damage dice rolled to have their highest value and that all attacks hit but do not crit.
• You can assume advantage on your attack rolls for the purposes of pricing abilities like the rogue's Sneak Attack.
• The damage must all be from the same source so for example you can't use Grasp of Hadar and Repelling Blast to move the target back and forth through a Wall of Fire or to push the target off of a cliff.
• This is single target damage only.
• This is for a single character by himself with no magic items for him or the target now nor in the past

Answer 1

Without setup you can deal at least 375 damage

The requirements:

We will need a Half-Orc Warlock (11) / Paladin (2) / Assassin Rogue (7).

These get us Savage Attacks, branding smite, Hexblade's curse, the Eldritch Smite Invocation, Divine Smite, Sneak Attack, and Assassinate. On top of this we need the Martial Adept feat and the Orcish Fury feat, and we will select the Dueling Fighting Style from Paladin. We will be using the booming blade cantrip.

On our turn:

On our turn we cast branding smite as a fifth level spell (5d6) onto our rapier (1d8). We cast booming blade making an attack (3d8), proc-ing Sneak Attack (4d6) as well as Divine Smite using a fifth level spell slot (5d8) and Eldritch Smite using a fifth level spell slot (5d8). We also use a maneuver gained through the Martial Adept feat (1d6), the Half-Orc's Savage Attacks (1d8), and the Orcish Fury feat (1d8).
All of these dice rolls (except Savage Attacks) are doubled due to the Assassinate feature.

Then we have the static modifiers: Our ability score modifier (5) and the Dueling Fighting Style (2).

Putting these together we have the following damage:
$$8 + 5 + 2 + 2\times(30+8+24+24+40+40+6+8) = 15 + 2\times(180) = 375\text{ damage}$$

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Note that this does not use hex nor Hexblade's Curse as these require bonus actions, and that is being spent on branding smite.

Answer 2

355 damage, or 467 with setup

The character

• Half-Orc for Savage Attacks
• 2 Fighter levels for Action Surge
• 2 Paladin levels for Divine Smite, searing smite, and 1 caster level
• 2 Divination Wizard levels for booming blade, absorb elements, Portent, and 2 caster levels
• 14 Forge Cleric levels for Blessing of the Forge, bestow curse, 3 ASI (Great Weapon Master, Martial Adept, Orcish Fury), Divine Strike, and 14 caster levels
• The total caster level is 17, so the Half-Orc has a 9th level spellslot
• Notable ability scores: 18 strength, 13 intelligence, 13 wisdom, 13 charisma

The turns

The order of operation isn't super strict but this is what you do:

Optional setup turn:

• Light a torch and inflict fire damage upon yourself
  • Cast an 8th level absorb elements
• Use Action Surge
  • Cast 5th level bestow curse chosing the last option

Damage turn:

• Cast 9th level searing smite
• Cast booming blade
  • Attack with a greataxe empowered by Blessing of the Forge
    • Use Great Weapon Master
    • Use Portent to foresee a critical hit
    • Use Orcish Fury
    • Use Divine Smite with a 5th level spellslot
    • Use Divine Strike
    • Use a maneuver from Martial Adept, such as Pushing Attack
    • Use Savage Attacks

The calculation

Curly brackets enclose damage derived by the optional setup turn.

portent × (greataxe + orcish_fury + searing_smite + booming_blade + divine_smite + divine_strike + pushing_attack {+ bestow curse + absorb_elements}) + blessing_of_the_forge + strength_modifier + great_weapon_master + savage_attacks

= 2 × (1d12 + 1d12 + 9d6 + 3d8 + 5d8 + 2d8 + 1d6 {+ 1d8 + 8d6}) + 1 + 4 + 10 + 1d12

= 355 {+ 112} damage with maximized damage rolls.
= 207.5 {+ 65} damage with average damage rolls.