Part of an ongoing series of 5e Rules notes.

In my most recent campaign, the Druid’s Moonbeam was colloquially known (well, at least by me) as the Orbital Death Laser of Doom.

It’s really not an exaggeration.

Let’s talk about Moonbeam

This is another one that has had some revising done on it since the original 5e release — so if you’re looking up material about it, make sure it’s referring to the “same” spell.

Here’s the official description.

A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high Cylinder centered on a point within range [120 feet]. 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.

Moonbeam, like Lightning …

This one has some similarities to Call Lightning, but some significant differences.

Like CL, the effect is a 5-foot radius centered on a grid intersection (i.e., 4 squares). The cylinder involved is 40 feet high, but there’s no verbiage (as in CL) to indicate that if you are not in a 40-foot tall room, you can’t cast it; the height seems primarily oriented toward cases of dealing flying creatures.

While CL allows the target radius to be moved anywhere under the cloud on the caster’s turn, Moonbeam‘s cylinder can be moved by the caster (as an Action) in any direction 60 feet, as long as it remains within the 120 foot range from the caster on that turn. So while someone can run out from under CL‘s cloud (which is immobile), the Moonbeam caster could run after someone who was trying to get outside of that 120 range and move the beam on top of them again.

Moonbeam and damage

This is a little more difficult to glean from the spell description, and had to be more fully explained in an official Sage Advice Compendium. Damage is done to victims within the radius on their turn, not on the spell-caster’s turn, i.e.:

• When they begin their turn inside the cylinder.
• When they enter the cylinder during their turn (if they did not already start there).

But targets do not take damage when:

• The cylinder is cast or moved onto the area where they are standing.
• The spell is moved across them on its way to a different location.

Looked at another way, you’d generally see Moonbeam’s use as such:

1. The caster drops the Moonbeam on an opponent. The opponent takes no damage at that time.
2. The opponent’s next turn begins … and they take their 2d10 radiant damage. They presumably move away …
3. The caster’s next turn starts — and they move the Moonbeam atop the same target. Who doesn’t take damage right then, but …

Note that victims can be involuntarily moved into the cylinder by some effect (a shove, a Thunderous Blast), and this does actually count as “enters the spell’s area for the first time on a turn.”.

Entering such an area of effect needn’t be voluntary, unless a spell says otherwise.

This same arrangement holds true for a number of other AoE hazards, e.g., Blade Barrier. If it moves onto you, you don’t take immediate damage. If you move into it (even against your will), you do. And, in such a case, you’ll end up taking damage twice — when shoved in, and then at the beginning of your turn.

Note that this, too, is a Concentration spell. Between that and having to use your Action to move the Moonbeam, use of this spell is pretty much a full-time job. But the orbital death laser’s damage potential, especially against mooks, or round-over-round against bigger targets, makes all that worth it.