So Hellish Rebuke inflicts 2d10 fire damage plus 1d10 for each slot level above 1st. Given fire resistance is the most common resistance, if one were to reflavour this with cold/necrotic/lightning for different entities/situations, should the alternate version be de-buffed a little due to the rarity of Resistance. Say 2d8 + 1d8/lvl - or are the frequency of resistances insignificant enough to warrant a de-buff for game balance?
A while ago, I got interested in this question and crunched a bunch of numbers based on resistances, immunities, and vulnerabilities of stuff in the Monster Manual. This is what I came up with:
| Bludgeoning | 0.997 |
| Radiant | 0.997 |
| Force | 0.997 |
| Piercing | 0.992 |
| Slashing | 0.990 |
| Thunder | 0.977 |
| Psychic | 0.969 |
| Necrotic | 0.942 |
| Acid | 0.926 |
| Lightning | 0.883 |
| Cold | 0.875 |
| Fire | 0.840 |
| Poison | 0.723 |
The number indicates how many hit points you can expect to remove, on average, for each point of damage dealt. So, if you deal 10 points of fire damage to a randomly selected creature from the MM, it will lose an average of 8.4 hit ponts. (Note that each actual target will lose 0, 5, 10, or 20, depending on whether it has immunity, resistance, no resistance, or vulnerability.)
How to apply this to spell damage is an interesting question. Casters often have more than one damage type available, and can guess the resistances or immunities of monsters, so it's not clear that you can simply divide one number by the other. However, it's not a bad approach for a first pass--and this is especially so for a spell like
hellish rebuke, where you get limited control of when and on what you can use it.
If we do use this approach on
hellish rebuke, which averages 11 fire damage at 1st level, we get that it should deal:
| Bludgeoning | 9.3 damage |
| Radiant | 9.3 damage |
| Force | 9.3 damage |
| Piercing | 9.3 damage |
| Slashing | 9.3 damage |
| Thunder | 9.5 damage |
| Psychic | 9.5 damage |
| Necrotic | 9.8 damage |
| Acid | 10.0 damage |
| Lightning | 10.5 damage |
| Cold | 10.6 damage |
| Fire | 11.0 damage |
| Poison | 12.8 damage |
Translating to die size, this suggests that a
hellish rebuke-like spell should use d12s if it does poison damage; d10s for lightning, cold, or fire; and d8s for anything else. (Acid is right on the edge, but it's
slightly to the d8 side if one goes a few decimals deeper.)
