For PCs, cantrip damage increases with their total character level and not their class level (see this question for reference: Do Cantrips use your character level or class level?).
However, NPCs do not normally have class levels, and their stat blocks don't state a total character level.
How can I determine the number of damage dice to roll for an NPC's cantrip?
- Should I use their number of hit dice as their total character level?
- Should I use their stated Spellcaster level? (What if they don't have one?)
- Some other method?
A couple of examples to illustrate my concern:
-
An Archmage (SRD p. 395) has 18d8 + 18 hit dice and is an 18th-level spellcaster, so methods 1 & 2 here would produce the same result.
-
Nezznar the Black Spider (LMOP p.59) has 6d8 hit dice and is a 4th-level spellcaster, so methods 1 & 2 would produce different results.
The fact that Nezznar has more hit dice than spellcasting levels implies to me that Nezznar has, in some unspecified way, multiclassed – and that 6 would be his total character level – and the number I should use for cantrips, not 4 (his spellcasting level).
A good answer would also cover NPCs with Innate Spellcasting and no spellcasting level. See, for instance:
- A Green Hag (SRD. p 319) has 11d8 + 33 hit dice but no spellcasting level listed.
Best Answer
Spellcasting
For the Spellcasting trait, you use the listed spellcaster level as read on page 10 of the Monster Manual:
This means an archmage is treated as level 18 for the purposes of cantrips, and Nezznar is treated as level 4 (despite his greater number of hit dice).
Innate Spellcasting
Innate Spellcasting will sometimes list a specific level (in which case use that one), but otherwise you use the monster's CR (as also stated on page 10 of the Monster Manual):
For the green hag, this means it is treated as level 3 for its cantrips.