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

Without in-combat setup you can deal at least 383 damage

The requirements:

We will need a Half-Orc Sorcerer (2) / Hexblade Warlock (9) / Paladin (2) / Assassin Rogue (7).

These get us Savage Attacks, Flexible Casting, 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.

Before combat:

Before combat begins we use the Flexible Casting feature to get ourselves a third fifth level spell slot.

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 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:
$$5 + 2 + 2times(30+8+24+24+40+40+6+8+8) = 7 + 2times(188) = 383text damage$$

Note that this does not use hex nor Hexblade's Curse as these require bonus actions, and that is being spent on branding smite. Also note that I did not use the Wild Magic Surge of 65-66 which causes "Up to three creatures you choose within 30 feet of you take 4d10 lightning damage" as it did not feel that really counted as damage from the cantrip.

If we allow for an extra setup turn we can deal at least 405 damage

We would use the same classes but would gain access to more options than before, in particular we can use Hexblade's Curse, and bestow curse from the Sign of Ill Omen Invocation.

This allows us to spend the setup turn casting bestow curse (1d8) as a fifth level spell (so it does not require concentration) and placing Hexblade's Curse on the target (6). Then on our actual turn we would cast branding smite and booming blade. Everything else would remain the same.

This would boost our damage by 6 + 2(8) = 22. So our new total would be 405 damage.

212 damage, or 260 with setup

The character

• Bugbear for Surprise Attack


• 9 Hexblade levels for eldritch blast, hex, Agonizing Blast, Hexblade's Curse, Sign of Ill Omen (grants bestow curse), and 5th level spell slots


• 3 Assassin levels for Assassinate


• 3 Sorcerer levels for Subtle Spell


• 2 Fighter levels for Action Surge


• Notable ability scores: 13 dexterity, 20 charisma

The turns

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

• Optional setup "turn":
  
  
  • Cast hex on the target using the Subtle Spell Metamagic. This happens outside of combat, while we are hidden.
  
  


• Initiate combat by surprising your target and roll a higher initiative than them so that Assassinate and Surprise Attack apply.


• Damage turn:
  
  
  • Cast bestow curse on your target choosing the last benefit. As a 5th level spell it does not require concentration.
  
  
  • Place the Hexblade's Curse on the target
  
  
  • Use the Action Surge to cast eldritch blast at the target
  
  

The calculation

Curly brackets enclose damage derived by the optional setup.

[# of beams] × ([assassinate] × ([eldritch blast] + [bestow curse] + hex) + [agonizing blast] + [hexblade's curse]) + [assassinate] × [surprise attack] = 4 × (2 × (1d10 + 1d8 + 1d6) + 5 + 6) + 2 × 2d6

= 212 + 48 damage with maximized damage rolls.

= 138 + 28 damage with average damage rolls.

341 damage, or 437 with setup

The character

• Half-Orc for Savage Attacks


• 3 Arcane Trickster levels for Sneak Attack, booming blade, and 1 caster level


• 2 Paladin levels for Divine Smite, Dueling Fighting Style, searing smite, and 1 caster level


• 4 Divination Wizard levels for Portent, 1 ASI (+2 Strength), and 4 caster levels


• 11 Gold Draconic Bloodline Sorcerer levels for Elemental Affinity, 1 ASI (Orcish Fury, Martial Adept), and 11 caster levels


• The total caster level is 17, so the Half-Orc has a 9th level spellslot


• Notable ability scores: 20 strength, 13 intelligence, 14 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
  
  

Damage turn:

• Cast 9th level searing smite
  


• Cast booming blade
  
  
  • Attack with a rapier
    
    
    • Replace the attack roll with a 20 Portent
    
    
    • Use Sneak Attack
    
    
    • Use Divine Smite with a 5th level spellslot
    
    
    • Use Savage Attacks
    
    
    • Use Orcish Fury
    
    
    • Use a maneuver from Martial Adept, such as Pushing Attack
    
    
    • Use Dueling Fighting Style
    
    
    • Use Elemental Affinity
    
    
  
  

The calculation

Curly brackets enclose damage derived by the optional setup.

portent × (rapier_dice + searing_smite + booming_blade + sneak_attack + divine_smite + savage_attacks + orcish_fury + pushing_attack + absorb_elements) + strength_modifier + dueling + elemental_affinity

= 2 × (1d8 + 9d6 + 3d8 + 3d6 + 5d8 + 1d8 + 1d8 + 1d6 + 8d6) + 5 + 2 + 2

= 341 + 96 damage with maximized damage rolls.

= 199 + 56 damage with average damage rolls.