My druid is prepping for next week's game and he's asked me if the Moonbeam spell will work inside the dungeon. I first interpreted the spells description…(PHB pg 261)
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.
… as meaning that the light shone down from the moon, or at least from the sky. Nowhere does it specifically say this though, and doesn't specifically mention that it can't be used indoors. The "silvery beam of pale light" could shine down from the ceiling as far as I'm reading it. I guess since there's no specifics about it then I go with the general, which is that it's a spell that can be cast wherever spells can be cast, but it seems wrong.
Am I missing something or interpreting this wrong, or does the Moonbeam spell in fact work indoors?
For reference (PHB pg 61)
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.
When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed
in ghostly flames that cause searing pain, and it must make a Constitution saving throw. It takes 2d10 radiant
damage on a failed save, or half as much damage on a successful one.
A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can’t assume a different form until it leaves the spell’s light.
On each o f your turns after you cast this spell, you can use an action to move the beam 60 feet in any direction.
Best Answer
If the Moonbeam spell required the actual moon to be shining down, and somehow focused its light (or similar), you would assume that its text would reflect that. No mention is made of this, nor of the restrictions that it would naturally apply to use of the spell.
This wouldn't just be a problem indoors - consider phases of the moon, daytime, cloudy nights, or even eclipses. In fact, it would be an extremely limited usage spell.
Contrast the Call Lightning spell (Thanks Tashio for providing the text):
This specifically says that it creates a cloud, includes limitations requiring you to be outdoors or have an incredibly high ceiling, and includes an additional effect if there is a pre-existing storm. Based on this, it seems clear that Moonbeam has no particular source or restriction whatsoever.
Obviously, you are the DM, and you are free to add fluff or requirements as you choose. But I would urge you to consider how limited this spell would become before you do.