How Is Damage Determined For Sunlight Hypersensitivity?

Question

So, as the title states, I'm curious about how the damage is determined with sunlight Hypersensitivity. I was looking through the monster manuals of DnD 5E and I came across a total of 4 monsters that have this: the Vampire, Vampire Spawn, the Bodak, and Vampiric Mist.

With the Vampire and the Vampire Spawn the damage for this is 20 radiant damage, but for the Bodak it's only 5 radiant damage, and for the Vampire Mist it's 10. I was wondering what determined this damage for each of these creatures? Is this based on HP at the average? Is it based on HP in terms of it being like for every 5 HP it's 1 point of damage or something? Is it based on CON score somehow? Is it based on Creature Type?

I had always assumed that the hypersensitivity was the same flat damage across anyone that had it, but it seems that something is determining how much damage is done.I'm curious because if it's something that can be fiddled with, I have an idea for something, but I can't figure out how this is done. Any and all help is greatly appreciated!

Answer 1

There is clearly not a uniform rule in place

If there was a relation to CR or hit points, the damage wouldn't be the same for the Vampire and Vampire Spawn. Many numbers in the monster stats are somewhat arbitrary as I found out when I did an in depth analysis of the whole Monster Manual.

Sunlight Hypersensitivity can be considered when determining CR for custom monsters

There doesn't need to be a uniform rule because the effect of the damage can simply be considered when determining CR. You can just take an expected probability that the monster will be exposed to sunlight in a combat and the damage and modify the amount of HP that you derive the CR from. Combats are assumed to take three turns. If you assume exposure for half the time and use 10 damage, treat your monster as having 15 less hit points when determining CR.