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Two examples.

A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high cylinder centered on a point within range. Until the spell ends, dim light fills the cylinder.

Moonbeam, Player's Handbook, pg. 261

You create up to four torch-sized lights within range, [...] each light sheds dim light in a 10-foot radius.

Dancing Lights, Player's Handbook, pg. 230

Now consider that each of these spells are cast on a bright day, obviously covered by Bright Light, as the game understands it.

  1. Do these spells change the Bright Light within the area affected by them into Dim Light?
  2. Is the behavior different between Moonbeam ("Dim Light fills the cylinder") and Dancing Lights ("each light sheds dim light"), or do both of them behave the same?
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  • \$\begingroup\$ Possibly relevant: rpg.stackexchange.com/a/152947/40516 \$\endgroup\$ – Ryan Thompson Oct 18 at 16:28
  • \$\begingroup\$ @RyanThompson I did see that question, yes, it's probably a good reference. \$\endgroup\$ – Xirema Oct 18 at 16:30
  • \$\begingroup\$ I can answer half of your question and I think I know the answer to the other half but I have no sources on it. Would it be worth posting an answer anyway? \$\endgroup\$ – Himitsu_no_Yami Oct 18 at 16:44
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Spells override Bright Light with Dim Light if the spell description says that it does.

Spells do what they say.

Dancing Lights states that "you create up to four torch-sized lights within range."

Moonbeam states that "until the spell ends, dim light fills the cylinder."

The light sources shed dim light, but nothing says this is a special type of dim light, or that the light is part of the spell's effects.

Moonbeam specifically indicates that the area of the spell is filled with dim light.

If the light from Moonbeam were overridden by the area being normally filled with bright light, this would make the spell's area invisible which seems counter to the intent.

So to answer the questions about your examples directly:

  1. Do these spells change the Bright Light within the area affected by them into Dim Light?

Moonbeam does, Dancing Lights does not.

  1. Is the behavior different between Moonbeam ("Dim Light fills the cylinder") and Dancing Lights ("each light sheds dim light"), or do both of them behave the same?

The behavior is different between the two spells.

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  • \$\begingroup\$ Aren't down votes are supposed to have a comment indicating what is wrong with the answer? \$\endgroup\$ – StarHawk Oct 18 at 17:31
  • \$\begingroup\$ Downvotes aren't required to be accompanied by a reason. If I had to take a guess, the downvotes are probably just people who don't agree that your interpretation is correct. \$\endgroup\$ – Xirema Oct 18 at 17:33
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    \$\begingroup\$ I've had a thought that might support the thesis of your answer. If you reread the description for Moonbeam, you'll note that the secondary effects, affecting Shapeshifters, are stated to apply "until it leaves the spell's light". If Moonbeam is not changing the light in its area of effect to Dim Light, then that part of the spell would have no effect in Bright Light. I don't know if this proves that it should be interpreted to mean it definitely replaces Bright Light, but it is a potential case that this is how this kind of effect should be interpreted. \$\endgroup\$ – Xirema Oct 18 at 17:37
  • \$\begingroup\$ @StarHawk As you wish. Downvoted for saying "Spells do what they say". \$\endgroup\$ – Mark Wells Oct 18 at 20:32
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    \$\begingroup\$ @Ruse Moonbeam says "Until the spell ends, Dim Light fills the cylinder", if it is replaced by Bright Light, Dim Light is not filling the cylinder until the spell ends. \$\endgroup\$ – StarHawk Oct 21 at 17:31
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Unless the spell, item, ability, etc mentions it specifically changing the level of light in a area/radius, then I would say no, it does not change it. Just think about turning on a dim flashlight outside during the day. It is still emitting light, but it doesn't make a difference. Now take that same light and go into a tunnel or cave and the dim light matters.

Here is an example of an ability changing the level of light:

Oathbreaker Paladin, level 20 ability

Dread Lord At 20th level, the paladin can, as an action, surround himself or herself with an aura of gloom that lasts for 1 minute. The aura reduces any bright light in a 30-foot radius around the paladin to dim light.

And a spell:

Shadow of Moil

The shadows turn dim light within 10 feet of you into darkness, and bright light in the same area to dim light.

Edit: to clarify though. For some spells or effects, such as the Twilight Domain's Channel Divinity, it is possible to be in the area of dim light from a spell/effect and be affected by it even if you are in an area with bright light. Because magic and D&D does not follow our worlds physics to a T.

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  • \$\begingroup\$ Thanks for the correction, I added it in. I think an area can, but would definitely be a case by case basis as to which effects take place. The Twilight's ability I used as an example mentions only being in the sphere created by the dim light, which I would rule works regardless of the lighting condition where as something like the Shadow Monk's Shadow Step probably not. Now an interesting interaction, if the Twilight Cleric's aura is active, but you are in bright light and the aura is invisible, you could have the enemy run in and harm themselves and not know what is going on. \$\endgroup\$ – DragoonKite Oct 18 at 17:29
  • \$\begingroup\$ Granted, if the player can do that, so can the enemy... \$\endgroup\$ – DragoonKite Oct 18 at 17:29
  • \$\begingroup\$ Also a valid point. \$\endgroup\$ – Xirema Oct 18 at 17:35
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Spells indicate if they reduce illumination

The illumination rules allow for multiple light sources, and one light source does not dim another.

Spells that reduce ambient light (as opposed to providing light) will say so. Examples:

  • Darkness says: "nonmagical light can’t illuminate it"
  • Tiny Hut says: "Until the spell ends, you can command the interior to become dimly lit or dark. The dome is opaque from the outside"

Moonbeam and Dancing Lights provide a light source, and do not say they reduce light. Spells like Holy Aura, Daylight and Sunbeam similarly state that they "shed dim light". These effects (and other light sources) are not 'invisible' in bright light, even if they don't alter local illumination levels.

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  • \$\begingroup\$ Does this mean that if moonbeam is cast in bright light, its area of effect is invisible? \$\endgroup\$ – mdrichey Oct 18 at 17:41
  • \$\begingroup\$ I think most DM's would adjudicate this modeling the situation after the real-world. If you look at DragoonKite's answer, they mention turning on a flashlight in daylight - the source of the effect is visible but doesn't shed any further light. That makes more sense for Dancing Lights than Moonbeam. \$\endgroup\$ – Joshjurg Oct 18 at 18:06
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    \$\begingroup\$ @mdrichey Bright, dim, and darkness are the 3 illumination levels with mechanical implications, but that doesn't mean that all sources of bright illumination are indistinguishable from each other, or that bright light makes dim light invisible. As with many things, the entire continuous range of illumination is cut into 3 discrete ranges for mechanical convenience, not realism. \$\endgroup\$ – Ryan Thompson Oct 18 at 23:04

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