So, as my title implies; If you shoot a Moonbeam at an enemy standing in a square, would the area covered by the Moonbeam be just that single square, from one of the corners 2x2 or mid-square 3x3 around the square with the target?
It originates from a corner
Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square.
As with most area spells, you don't target an enemy with it, just a patch of ground. So it covers 2x2 squares.
Moonbeam Would Be 2x2, Originating at a Corner
The DMG (p. 251) establishes the convention of placing spell area of effects at the intersection of two squares. DMG 251:
Choose an intersection of squares or hexes as the point of origin of an area of effect...
The rule doesn't say who should choose, but I let the player who cast the spell choose.
Xanathar's Guide to Everything also references this point of origin placement rule under the heading Areas of Effect on a Grid, page 86.
Keep it Quick and Easy
When I use a grid I like quick and easy. Personally I use the Template Method rules found in Xanathar's Guide to Everything (p. 86):
Get a piece of paper or card stock, and cut it in the shape of the area of effect you're using.
To use an area-of-effect template, apply it to the grid ... If any part of a square is under the template, that square is included in the area of effect. If a creature's miniature is in an affected square, that creature is in the area.
This method is quick, easy and versatile:
- You do all the work up front and then it can be applied in seconds during game play.
- The rules are simple and you don't have to decide if a square is half covered or not - which by the DMG (251) rules you do need to do.
- If the square is fully or partially covered, a creature in that square is in the area of effect.
- You can even use this if you're using miniatures without a grid, though I usually judge more leniently in this case
For circular areas of effect I poke a hole in the center and stick a pencil in them. Then I can place the point of the pencil directly on the center point of the spell area of effect.
Creating a Template
I use 1 inch grid paper for measurements and a compass, or you could use one of these methods, to draw the circles. For the Moonbeam spell, which has an area of effect of a circle with 5 foot radius the circle will have a radius of one square. Larger areas of effect use larger template circles.
I like to have the grid on the AoE template because that makes it even easier to visualize which squares are in the AoE. Usually I paste or draw a picture of a fireball or other spell on the back side and the player can flip it for for dramatic impact.
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.
The Player's Handbook says, p 204, when casting a spell with a cylindrical area of effect:
A cylinder's point of origin is the center of a circle of a particular radius, as given in the spell description.
If you shoot a moonbeam at an enemy, and you choose the point of origin to be where the enemy is, then the center of the circle is where the enemy is standing.
Additionally, the DMG on p249, under Adjudicating Areas of Effect, says in part:
Many spells and other game features create areas of effect, such as the cone and the sphere. If you're not using miniatures or another visual aid, it can sometimes be difficult to determine who's in an area of effect and who isn't. The easiest way to address such uncertainty is to go with your gut and make a call.
Using a Grid
It sounds like you're using the optional grid rules from the PHB, p 192.
There are some challenges to using the grid system.
If the point of origin of the circle is where the target is, then it is where you decide the creature is. Is the creature in the center of the square? Then that's where the moonbeam is centered. Then the issue is how to handle the surrounding squares.
The DMG on p 251 in the section on Using Miniatures talks about additional rules to the grid system, and says that under this method the point of origin is restricted to a grid intersection.
The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren't. Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square.
To summarize, assuming you want to use a grid system to visualize combat, you're going to have to make some compromises. Round pegs don't fit into square holes very well. Round areas-of-effect don't fit a grid system very well. You can center the AoE in the middle of a square, but then you have squares that are partially covered and you're going to have to adjudicate that. Or you can place the AoE at an intersection, but then you're denying the spellcaster the ability to actually target where the target is standing.
In the end, the GM is going to have to adjudicate exactly how it works in their game.