The game presents 10 damage types: acid, cold, fire, force, lightning, necrotic, poison, psychic, radiant, thunder.
And my players asked a few things about using spells that deal typed damage in and out of combat:
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Does Psychic damage work on undead or on creatures with Int or Wis 1 or less?
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Radiant damage is basically a divine light, so in physics terms it's infrared rays. Can it be used to "toast" bacterias and purify (non magic) water?
Best Answer
No creature is immune to any kind of damage unless its stat block explicitly says it is.
You can think of psychic damage as interfering with a creature's "nervous system" or whatever magical thing it uses as an equivalent. Even mindless undead have some way of perceiving their environment and then manipulating the various parts of their body to get their body over to it and attack it; psychic damage is essentially high power feedback (like the really loud screech when you get a microphone next to its speaker) on these channels. If something would be better classified as a trap than as a creature, then it's probably immune to psychic damage.
Radiant damage isn't really light so much as it is divine power. It happens to look like light, but it's not; a laser or high-intensity infrared source would do fire damage. Radiant damage is like a flashlight that's been imbued with holy power to accomplish a specific task (namely, destroying some poor creature); the holy power only does what the ability says it does (damage), so powers that do radiant damage can't be used for other purposes unless blinking a bright flashlight briefly could accomplish those same purposes.
A brief discussion of the other damage types: