The Moonbeam spell states:
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 of your turns after you cast this spell, you can use an action to move the beam up to 60 feet in any direction.
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d10 for each slot level above 2nd.
Last session, the party was fighting a Froghemoth and the monster swallowed up 2 party members (the Froghemoth bite action states the swallowed creatures are now in total cover). The cleric was casting moonbeam to damage the Froghemoth. I understand the you must have a clear path to target something with magic, but in that case, the spell has been cast already, and its description says the damage happens to a creature that enters or starts its turn in the spell's area. The area is already active in that case…
Should the characters inside the Froghemoth take damage as well on their turns?
Best Answer
The swallowed characters take no damage
If you are affected by a persistent Area-of-Effect spell after the casting depends on the individual spell. In the case of moonbeam, the spell states
So in this case it's clear that the effect is an ongoing emanation from a point within range, at the top of the cylinder 40 feet up. The general rules for spell effects on page 204 PHB state:
Because in the case of moonbeam, the effect continues to emanate from a point of origin, anything that is behind total cover will be excluded from the spell's area, and the creatures there do not suffer its effects.