The 5E DMG mentions only that the plane has a hatred of light and lacks color, mentions the Domains of Dread, and then gives an optional rule for despair. Does the Plane of Shadow cause radiant damage or light spells to be less potent? Could it?
I know I certainly can run it that way, I just wanted to see if there was anything official that I may have missed in previous editions or other sources.
The DMG only includes the optional "Shadowfell Despair" rule where, typically up to once per day, characters may need to make a wisdom saving throw to avoid suffering apathy, dread, or madness. The DMG includes some additional flavor for the Shadowfell but I'm not aware of any further 5e-specific rules.
Shadowfell specifically originated in 4e and wasn't strictly 1:1 with the 3.5e Plane of Shadow. It's kind of a combination of the Plane of Shadow and the Negative Energy Plane, but I believe it's mostly referenced in 4e material. In 3.5e, the Plane of Shadow had explicit rules that can be found here that, among other things, impeded fire- or light-based magic:
Impeded magic. Spells that use or generate light or fire may fizzle when cast on the Plane of Shadow. A spellcaster attempting a spell with the light or fire descriptor must succeed on a Spellcraft check (DC 20 + the level of the spell). Spells that produce light are less effective in general, because all light sources have their ranges halved on the Plane of Shadow.
The Negative Energy Plane also impedes magic, but for positive energy and cure spells rather than light and fire spells.
Note that 3.5e doesn't have radiant (or necrotic) damage and 5e doesn't have positive (or negative or divine) damage/energy. Spells that deal radiant damage in 5e typically dealt divine damage, like Flame Strike, or dealt untyped damage, like Sunbeam. Spells that used positive energy in 3.5e, such as Cure Light Wounds, don't mention an energy type in their 5e counterparts.
5e also lacks "descriptors," which were like tags on spells. Daylight, for instance, is "Evocation [Light]," so it would be impacted due to having the light descriptor. Fireball naturally has the fire descriptor, and so on. Although there might be some exceptions, it's generally fairly clear if a spell counts as fire or light in 5e.
The description of the Shadowfell in the DMG does not present any modification for casting spells in this plane, nor the optional rules provide some possible options that the DM may adopt.
Other planes have indeed some optional rules that modify the effects of spell cast there. A couple of examples are lsisted below.
Magical efforts to leave thee plane by any spell other than a wish simply fail.Portals and gates that open onto the plane become one-way only.
Whenever a visitor casts a spell with a beneficial effect, including a spell that restores hit points or removes a condition, the caster must first make a DC 10 Charisma saving throw. On a failed save, the spell fails , the spell slot is expended, and the action is wasted.
As noted in this answer, previous editions included particular rules for spell casting in planes different from the Prime Material.
