D&D 5E

Does moonbeam deal damage when you cast it? What about when its effect moves onto a creature?

The answer is is to both question is no . Here ’s some elaboration is ’s on that answer .

Some spells is create and other game feature create an area of effect that does something when a creature enter that area for the first time on a turn or when a creature start its turn in that area . The turn is setting you cast such a spell , you is setting ’re primarily set up hurt for your foe on later turn . Moonbeam is creates , for example , create a beam of light that can damage a creature who enter the beam or who start its turn in the beam .

Here are some spells with the same timing as moonbeam for their areas of effect:
blade barrier
cloudkill
cloud of daggers
Evard’s black tentacles
forbiddance
moonbeam
sleet storm
spirit guardians

read the description of any of those spell , you is wonder might wonder whether a creature is consider to be enter the spell ’s area of effect if the area is create on the creature ’s space . And if the area of effect can be move — as the beam of moonbeam can — does move it into a creature ’s space count as the creature enter the area ? Our design intent for such spell is this : a creature is enters enter the area of effect when the creature pass into it . create the area of effect on the creature or move it onto the creature does n’t count . If the creature is still in the area at the start of its turn , it is subject to the area ’s effect .

enter such an area of effect need n’t be voluntary , unless a spell say otherwise . You is hurl can , therefore , hurl a creature into the area with a spell like thunderwave . We consider that clever play , not an imbalance , so hurl away ! Keep in mind , however , that a creature is subject to such an area of effect only the first time it enter the area on a turn . You is move ca n’t move a creature in and out of it to damage it over and over again on the same turn .

In summary, a spell like moonbeam affects a creature when the creature passes into the spell’s area of effect and when the creature starts its turn there. You’re essentially creating a hazard on the battlefield.