PHB pg. 261, Moonbeam:
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...
On each of your turns after you cast this spell, you can use an action to move the beam 60 feet in any direction.
We read this to mean a creature takes damage from moonbeam when any of the following conditions are met:
- It begins its turn inside the moonbeam
- It enters the moonbeam on its turn
- The creature finds itself inside the beam on another character's turn, no matter how it got there
The creature will not take damage in the following instances:
- A creature moves from one part of the beam to another (obviously it had to take damage, but it won't take more damage)
- A creature is in the beam after having been inside the beam already on that turn.
Following these assumptions, our druid cast a moonbeam and did damage to two enemies, Bob and Frank. On their turn, they took damage again, killing Bob. Frank left the beam. On the druid's next turn, she moved the beam in a circular motion, not exceeding 60 total feet of movement, and in the process the beam came in contact with a dozen enemies and ended on Frank. All dozen enemies received damage, as they were inside the beam at some point during the turn. On Frank's next turn, he again took damage for the fourth time and died. On each of the druid's turns, she moved the moonbeam around like this until the fight ended (it did not exceed the time limit and she made all concentration checks).
Is this a legitimate use of Moonbeam? It seems awfully powerful for a level 2 spell.